Question about switching to synthetic oil

My 1994 SC has around 122k and I want to switch to Mobil 1 synthetic oil. Is this a good decision or not? I want to start modding my sc and want to run synthetic. The rear main seal already leaks and I am getting that done fairly soon.

In my opinion I don't see a problem why you couldn't switch to synthetic oil. Personally I run Mobil 1 Fully Synthetic, 15W-50. Hope this helps.

There are many opinions and lots of theory out there. I won't profess to know the answer to your question but I will give you my take on it.

Many people have reported that synthetic oil will cause or at least aggravate leaks on an old engine.

It has also been said that because of it's more uniform molecular structure, a motor that has always been run on synthetic will have tighter bearing tolorances. And conversely a motor that has been run on regular oil will have looser tolerances. Running a comperable weight (like 10-30) synthetic on a motor that has been run on standard oil may result in reduced oil pressure and sloppy bearings.

My advice to you is if you decide to run synthetic, ONLY run 15-50. Do not run 5-30. Expect leaks. If you decide to stay with standard oil, use a good quality brand and change it often. In any case I recommend changing oil when it gets dirty. Some motors may require this often, others may not. If you use standard oil, I do not recommend adding oil between oil changes. If a motor is using oil, I take that as a sign to change the oil more often. When it gets a quart low, change it and don't waste your money on synthetic oil if you are running a quart low in less than 3000 miles.

In any case, since you really don't know the condition of the motor, I recommend that you get a real oil pressure gauge and monitor oil pressure. A good SC motor will get 75psi cold, 40psi hot idle, and 65 hot cruise. Keep that in mind as you run and/or mod the motor. SC's are really bad about eating up rod bearings any time after 75K miles.

Hope that helps, the decision is up to you.

my sc has 145k on it and the mechanic that i go to up here, very knowledgable on the sc , told me that swithing to synethetic now could very possibly damage lots of internal parts....he explained it to me kind of like a trans if you dont get the trans fluid changed regularly and you go a very long time with out doing so, when you do change the fluid you have a very big chance of the trans slipping because the trans gets used to shi**y fluid....just thought i would give you some insight on what my mechanic told me....hope it helps

It's anybodies guess. My brother has a Mustang LX 5.0 that he bought with 70,000 miles on it. He changed the oil when he first bought it and just added oil when it needed it. Never changed it again. Well, he retired it at 350,000 miles a week after running a low 14 at the track. The theory at my work is that if it is leaking oil, that means there is still some in there. It's when it stops leaking that you should worry. My other brother bought a 85 Mustang with 180,000 miles on it and put synthetic in it and lost the rear main seal within a week. So you never know. This doesn't help much does it.

Darkside said:

It's anybodies guess. My brother has a Mustang LX 5.0 that he bought with 70,000 miles on it. He changed the oil when he first bought it and just added oil when it needed it. Never changed it again. Well, he retired it at 350,000 miles a week after running a low 14 at the track. The theory at my work is that if it is leaking oil, that means there is still some in there. It's when it stops leaking that you should worry. My other brother bought a 85 Mustang with 180,000 miles on it and put synthetic in it and lost the rear main seal within a week. So you never know. This doesn't help much does it.

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Remind me not to fly in an airplane you serviced.

i was at 140,000 miles when i switched to full synth. mobil 1. man, that stuff is expensive but i've driven the car to jersey and back which is about 900 miles one way, race it occasionally, and took it to the shootout, 500 miles one way, raced, dynoed, drove back and so far, no leaks or anything like that. maybe i got lucky, but the car is running great (knocks on wood).

XR7 Dave said:

Remind me not to fly in an airplane you serviced.

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I don't fly in airplanes I service! Pilots always ask us to go on test flights after we have performed major service on their aircraft. I usually tell them "What, are you crazy?! I won't fly in these things!!" You think rebuilding SC engines is stressful, try watching a pilot take off in an airplane that you just replaced an engine cylinder in. You start to wonder, "did I hook up that fuel line?", "did I torque all the nuts correctly?", "where did I leave my 3/8 socket?" , you feel alot better once they land the thing and everything is OK! I mean, if someone were to crash and die because I didn't do something correctly, I would go to jail.

XR7 Dave said:

There are many opinions and lots of theory out there. I won't profess to know the answer to your question but I will give you my take on it.

Many people have reported that synthetic oil will cause or at least aggravate leaks on an old engine.

It has also been said that because of it's more uniform molecular structure, a motor that has always been run on synthetic will have tighter bearing tolorances. And conversely a motor that has been run on regular oil will have looser tolerances. Running a comperable weight (like 10-30) synthetic on a motor that has been run on standard oil may result in reduced oil pressure and sloppy bearings.

My advice to you is if you decide to run synthetic, ONLY run 15-50. Do not run 5-30. Expect leaks. If you decide to stay with standard oil, use a good quality brand and change it often. In any case I recommend changing oil when it gets dirty. Some motors may require this often, others may not. If you use standard oil, I do not recommend adding oil between oil changes. If a motor is using oil, I take that as a sign to change the oil more often. When it gets a quart low, change it and don't waste your money on synthetic oil if you are running a quart low in less than 3000 miles.

In any case, since you really don't know the condition of the motor, I recommend that you get a real oil pressure gauge and monitor oil pressure. A good SC motor will get 75psi cold, 40psi hot idle, and 65 hot cruise. Keep that in mind as you run and/or mod the motor. SC's are really bad about eating up rod bearings any time after 75K miles.

Hope that helps, the decision is up to you.

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Right now I use Mobil 20W-50 non synthetic oil, what would switching to Mobil one 15w-50 do? Is it that much of a difference?

Add another vote for synthetic. My ol' man and cousin are/were both mechanics and basically said I'd be a fool if I didn't switch especially with a high output engine. It makes a huge difference in cold climates too.
As far as leaks, the way I figgure it is if it's leaking something ain't right so using synth will help point out where a problem is.

I've heard at least one mechanic suggest, when switching from regular oil, to run a synthetic blend for a couple of oil changes before running a full synthetic. The theory is that it lets seals and such "get used to" the synthetic. Now, I'm 99% sure this is pure voodoo, but hey, if it makes you feel better, go with it.

You can run a full synthetic no problem..I used to run 15/50 but now use the 10 or 15/30 depending on brand..Once the oil is hot even a 30 weight synthetic will have better viscosity then 50 weight organic oil...Plus you do get dyno proven power from synthetic;O)

i bought mine with 99k on it after it was sitting for almost a year. first oil change i did was valvoline synthetic 10-30 about a week later it was dirty so i changed it again, so far since i got it back running its had 3 oil changes in 5 months. the oil is now starting to stay cleaner longer. it seems to me that the sythetic is actually cleaning out the motor, call me crazy but what else explains it. i have not noticed any leaks from all 3 of my birds that were running synthetic. the same thing happened when i bought my first lx, and i sold it with 160k on it and was running like a dream, with synthetic for the last 60 k i had it.

Synthetic does have high detergent properties which is also why it can cause leaks. If bearings already have large clearances (an absolute gaurantee with an SC motor) then a thin oil will not provide the required oil film that is needed to float the loose bearings.

Is full synthetic 5W-30 ok in a newly rebuilt SC motor?

Joe generally youll want to put about 1,500 miles on yer new motor to break in rings before using synthetic...

DamonSlowpokeBaumann said:

Joe generally youll want to put about 1,500 miles on yer new motor to break in rings before using synthetic...

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And then the answer would be yes.

The debate on oil continues. I run 5w30 Mobil 1 in both my SC's ('93 has 200,000m & '94 has 120,000m) during the winter months (say December to March in the NE) and 10w30 the rest of the year. No leaks and I don't add any oil between 3-4000m oil changes. And I've run these motors in the 130-150mph range lots of times. I was always led to believe that going beyond the 25 points weight spread, in either conventional or synthetic, meant that way too much polymer was added to the base oil (in order to provide that extended viscosity) and that weakened the protection, particularly for synthetics. That's why a lot of the new car manufacturers will void warranties if people even use 10w40. So why would you use 15w50 or 20w50 except maybe for racing.

Good question

I have a new rebuilt engine too, and was going to put in 10w-30 Royal Purple(is that okay)??...I had another rebuilt engine went 5,000 miles on regular oil changed it to 10w-30 Mobil 1 and the crank cracked 2,000 miles later going 35 mph.....What to do?? Which weight??

thanks.

I don't think there is a debate on whether or not synthetics are better, but rather is it right for your application?

Personal experience is never the best example. There are too many underlying factors to trust personal experience. That is why I have not relayed my personal experience nor would I ask anyone else to follow in my footsteps.

However, I have it on pretty good authority (I've read a lot) that oil and bearing lubrication is a lot more than just friction properties and heat protection.

Generally accepted truth:

Sythetics have fewer impurities in the oil itself. I'm not talking about dirt, I'm talking about parafins and other non-lubricating compounds. The molecular structure of sythetics is also far superior to standard oil. There is absolutely no question as to the better lubricating qualities of synthetics.

Synthetics, because of their pure and consistant molecular composition, will flow into smaller clearances and resist being wiped off better than a conventional oil.

Synthetics will not form sludge as quickly as conventional oils. Sludge is caused by the by-products of combustion getting into the oil. The detergents in the oil are intended to suspend these particles and prevent them from grouping together and forming deposits. Due to the molecular composition of a standard oil vrs a sythetic, the detergents in the synthetic oil are able to perform their job many times better and for longer times than the conventional oil.

Because the synthetics do not have the large amount of parafins floating around in the mix and the molecular structure of the oil itself is more consistant, synthetics are capable of running at much higher temperatures than conventional oils.

Now there are other "truths" as well but that is enough for starters.

What does all that mean to me?

Well, that is where the interpretations start to come in. I will post some ideas and how these things *may* relate to your experience. These are opinions, and most of them are not MY opinions, just ones that I feel have validity in helping you to decide what oil to run in your motor.

Due to the increased detergent capabilities of synthetic oil, you will find that this oil will "attempt" to clean out your engine. I say attempt, because it is also fact that some engines have so much carbon and varnish deposits in them that NO oil will ever clean them out. It may be better to leave the deposits where they are than to try to clean them out with your oil. Remember, once a filter becomes clogged it bypasses. The oil will not stop cleaning. So you may inadvertantly be forcing carbon deposits through your already worn bearing clearances. Be warned. On the other hand if your motor is pretty clean before switching to synthetic, expect the oil to get dirty pretty quick the first couple of times the oil is changed, then start to stay clean longer. This is evidence that it has done it's job.

Also due to the cleaning abilities of a synthetic oil, you may notice leaks forming. This is possible for two reasons. First off, old gummy oil and varnish deposits may actually be partially sealing some leaks. The high detergent properties of the synthetic oil will find them if they exist. Second, since synthetic oils (especially the light weight ones) have a smaller and more consistant molecular structure, they will seep into cracks that normal oil would not get into. These are two reasons why a motor *may* leak once a synthetic has been used. Switching back to conventional oil won't likely fix the problem.

Then there is the issue of bearing wear. It has been stated that a motor which has operated since new on synthetic oil (particularly a lightweight one) will NOT wear as much as a motor which has been run on conventional oil. You have to understand here that there is a critical wear period on a new motor and then normal wear over it's life. During the initial critical wear period, all the bearing clearances will establish themselves at a "happy medium" based on the friction properties of the oil. If you use a heavy weight conventional oil in a motor in these initial stages, the clearances will open up to accommodate the properties of that oil. Conversely, if you run a lightweight synthetic during this time, the clearances will not open up as much and the motor will retain tighter bearing clearances. What happens in this initial period (not talking about 500 miles, more like 10,000 miles) is that all the parts are polished to those clearances and for the most part the surfaces of both the bearings and crank journals are somewhat sealed. Which means that they will no longer "adjust" quickly or easily to changes in lubrication requirements. So what you ask? The point is that if a motor has tight clearances and you start using 20/50 oil because you are going to race the vehicle, you may find that the tight clearances (which after 100K+ miles are SET in the motor) cause the bearing surfaces to over heat and the heavy oil can actually trash the bearings.

Now, look at in on the flip side. Let's say someone used heavy oil in the motor over the years and now you want to use 5w30 synthetic in it. So the motor already has large clearances, and is actually on the edge of wearing through the bearing material, and you put in a lightweight (albiet excellent quality) oil. Now, due to the large clearances, there is not enough oil pressure to float the bearings. *PLEASE NOTE* Engine bearings do NOT slide smoothly on the crankshaft surfaces!! They are NOT supposed to touch at all. A film of oil is supposed to flow between bearing surfaces at ALL times. No oil can protect the crankshaft from constant contact with the bearing surfaces. So if your fancy synthetic oil does not have enough viscosity to maintain sufficient oil pressure and hence isolate the crank from the bearing surfaces, the bearings WILL fail. An old rule for oil pressure is 10psi/1000rpms. Keep in mind that the oil requirements of this motor and the output of our oil pumps are NOT linearly matched. Therefore it is not possible to have 10psi oil pressure at idle and have enough pressure for high rpms. A good SC engine will have 25psi (or more) hot idle and 60psi (or more) hot above 2500rpm. Note that to take the motor past 5000rpm you better have at least 60psi or you are playing with disaster.

So, you can see that lubricating qualities are not the end all for oil and bearing protection. If you have a high mileage motor and you want to run light weight synthetics in it, then at bare minimum get a mechanical oil pressure gauge ($15 at AZ) and mount it under the hood so you can monitor oil pressure. If your motor doesn't have the oil pressure you want to see, USE A HEAVIER OIL.

There are other reasons for using a heavier oil. Under normal conditions, the internal operation of a motor is a very smooth chain of events. It's a beautiful thing really. And that is how the motor is supposed to operate. However, in the real world, such is not always the case. Our cars have one NASTY tendancy that throws a monkey wrench in all of that. It's called detonation. It's ugly when gasoline explodes rather than burns smoothly as intended in an engine. Our motors are prone to detonation. The extreme violence of detonation is just really hard to wrap your mind around and totally comprehend, but trust me when I say that it can melt pistons, bend connecting rods, and break crankshafts. What is my point? Well, think of that little thin layer of oil between the connecting rods and the crank as a tiny little itty bitty cushion. On a motor that is prone to detonation, I like the idea of a "firmer" cushion. This is why I run a heavier oil than necessary in my racing SC motors. I didn't think this idea up, but it sure made sense to me.

And finally, a little common sense that I came up with myself. :\

1) If your motor is running rich, and therefore contaminating the oil rather quickly, there is NO point in running synthetics in it. You migh just as well change the oil with standard oil every 1500 miles. No oil is going to be effective when diluted with gasoline.

2) If your motor is using oil there is no point in running synthetics. If the motor is burning oil, that means it is getting drawn into the combustion chamber either past the valve guides or the rings. If oil is going in, gas and or carbon are coming out. It's a two way street. If the oil is becoming contaminated, you'd best get it out of there. Not to mention it makes for an expensive fuel additive.

3) If you find that you can run synthetics successfully, then do yourself a favor and change the filter every 3000 miles regardless, and change the oil every 12K miles or once a year. I have found that a good engine may use only 1 qt every 5000 miles and the oil never really gets black.