Grinding gears with synthetic trans fluid
 

Sorry for the length of this post but I'm stumped. Son just bought an '86 GXL NA with manual trans 114Kmi. Car was garaged in mint condition and was apparently well cared for. Car drove well with clean shifts and no problems with gear changes even thru redline revs. After three weeks of clean driving, we decided it was due for trans fluid change so drained old and put in Amsoil synthetic 75w-90w gear lube. Got in car and proceeded to test the synthetic trans fluid's reputation for smoothness and performance. After about 20 mins of normal to hard driving it suddenly started to grind on the 1st to 2nd shift when over 5K rpm. Not huge grind, just a little burp before it goes into gear. This progressed to a 2nd to 3rd gear shift burp and a louder nastier 1-2 grind. Since the only thing that changed was the fluid, we drained it and refilled it with Castrol Mineral (non-synthetic). Same thing after 10-20 mins of driving. Pedal freeplay has been adjusted to spec and master /slave appears ok. Is it possible that since the synchro needs friction against the gear to work, the oil made them 'too' slippery????? Is the synth's coating still on the gears even though it's been drained? I've read posts where other owners have used synths to eliminate grinding synchros but haven't read any where synths CAUSED grinding. Anybody run into the same thing or have any ideas on a fix? Thanks in advance.

I haven't heard od that, but you may try putting some non-synth in there. i can't think of anything else right now other than bads synchs or throwout bearing or something, assuming the master cylinder is ok.

I'd stick with the regular type oil and hope it goes away. I'd bet your problem was that you didn't have quite enough friction with the synthetic...

Brian

When I switch to Redline MT-90, it took a few weeks (only drive on weekends) to get the smoothness back, I thought it was rather weird too!?!?! Now it's almost like shifting in butter :D

When you switch to a synthetic gear lube, it is very important to use one that is specifically formulated for transmissions. I was researching putting synth in my Triumph and heard many horror stories just like this one about running synth in a tranny. I used Redline MT90 and my little Spitfire has never shifted better. I used their 75-90 gear oil in the diff and it works great, but I was warned that any synth gear oil that was not specifically for trans will most likely defeat the action of the synchros.

Just my $.02

Have replaced with Stalube mineral gl4 to eliminate the additives for gl5 specs. These additives are for lim-slip diffs and can eat at synchros. Drove it and still had problems. Then went extreme and replaced it with straight Valvoline 30w motor oil (yea I checked it's ok for short term) and am driving it to help flush out synth gl5. Improved--it will let me shift at higher rpms without forcing but still not all there yet. Will follow RETed's suggestion and allow a little time and then change to Redline MT90.