4 Speed gear oil

I tried Mobil 1 synthetic gear oil and the tranny would grind on every shift. It lasted for a round the block drive and I drained and replaced with 80w/90 non synthetic and it was fine. I have heard this same experience from a lot of folks. Likely the limited slip friction modifiers that come in it don't let the syncros grab resulting in the gear crunch.

I have tried ATF and the shift quality is much proved over the gear oil but you do get a lot rattle from the gears in neutral. I tried Penzoil Syncromesh Fluid a couple of years ago and it's better than the ATF without the noise.