I chewed up a set of brass blockers by speed shifting. If you're going to race the fiber type are better. I used Mobil 1, 5W-30 with them and the Tremec rep at the dragstrip said he thought it was a good fluid for manual trannies. Honda/Acura has an excellent manual transmission fluid, too, as is GM's Syncromesh but it's expensive. With the fiber blockers I'm running Valvoline synthetic blend Mercon III/V ATF. Mainly because that's what PEP Boys had in stock when I bought it. I don't know how the fiber blockers stand up to other fluids so I'm sticking with the Mercon ATF.