Same problem
I'm having the same problem with my '90 SC. The dipstick and tube are from my original '90 block. But I'm using a '94 block with a stock '94 crank, stock 94 rods and 20 over forged flat top Diamond pistons.
I pulled the dipstick out one day and a section from just above the full mark to the end was missing. It's probably still in the bottom of the pan. Purchased a new dipstick and now, a couple of thousand miles later, it has a wear mark on the side of it just above the full mark. Something is rubbing against the dipstick.
I know the tube is fashioned to the correct exhaust manifold bolt.
Since this engine was built, I've had to put 5.5 quarts of oil in it to show full on the dipstick. That's with a filter change, adding 5 quarts, starting it up, shutting it down and checking again and then adding a half quart so it shows full on the dipstick. That, in itself, doesn't seem right.
I'm planning on putting the broken dipstick in the tube and leaving the good one in the trunk.
eddie
'90 5-speed SC
'97 LX sport