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.

 

It's always possible that they just now started to wear and it has nothing to do with the oil. The car does hav alot of miles

 

I know swapping to synthetic oil in the engines of old cars can cause problems, because the synthetic clears out all the old crud, that's very often filling up gaps and making things work. I couldn't tell you whether a similar thing is possible in a tranny though.

Charlie

 

Redline MT90 is supposed to help this. If you read Redline's advertising they say that the syncro's do require a certain amount of friction to work properly.

Rob

 

i'm going to try redline to attempt to quiet my grinding. so i'll be able to tell you in 2-3 weeks. redline is supposed to be good stuff. expensive too unfortunately. we'll see.

 

you have worn out syncro's. Changing the Synthetic will only provide fresh fluid for the worn out syncro's.

Do a search. There is several threads that get into the nity grity's of types of Tranny fluids that work best for our Rex's. The wrong kind will absolutly shorten the life of your syncro's (learned this myself the hard and expensive way).

Anyway, I'm betting you have a tranny rebuild in your future. The NA tranny's tolerance was barely within spec for stress from a stock 13B engine anyway.

 

Was that "Amsoil synthetic 75w-90w gear lube" GL-4 rated or explicitly stated okay for use in modern synchromesh transmissions???

If it was straight gear oil (GL-5), then the sulfate additives will attack the brass synchronizes, causing your problem.

"Transmission oil" needs to be specific.


-Ted

 

I have caused grinding using synthetic Mobil1 gear oil. Still use and recommend it for frontends/rearends etc.

I know that the Redline MT90 is synthetic/GL4 and never had problems with it. I also have used/installed the Amsoil products with no problems.

Did you get the right Amsoil?
Notice that of the 5 gears lubes, Amsoil has a separate GL4 MT1 oil(3rd one in list of the 1st link):
http://www.amsoil.com/products/agr.html
http://www.amsoil.com/products/tgr.html

I would avoid any gear oil that does not specifically state that it can be used safely in manual transmission that required GL4 gear oil.

Notice that RoyalPurple mentions that in their Product Sheet:
http://www.royalpurple.com/retail/pr.../maxgear.shtml

Notice that Redline mentions this:
http://www.redlineoil.com/redlineoil/mtlti.htm

Notice that Neo says that it is safe for gl4 manual trannies:
http://www.neosyntheticoil.com/75w90hd.htm

Notice what Pennzoil says concerning gl5 corrosion:
http://www.pennzoil-quakerstate.com/...s/pdf/p21b.pdf

 

Ok, here's an update.
Already understand with 114k miles on the clock that trans is worn but the coincidence of grind IMMEDIATELY after oil change is just too great.
Had done all the searches before posting since I didn't want to waste anybody's time but as I stated in original, had never seen anything on adding synth to cause gear grind, just to relieve it. Amsoil themselves recommended the type we used, Series 2000 75w-90w gl2-gl5. Specifically states backward compatible for gl4 applications. Since problem, we've tracked down specific info on differences between gl4 and gl5. RETed is correct about the extra sulfate additives in gl5 attacking brass synchros (OUCH) and also are added to lower friction coefficient and increase pressure resistance. Additives are necessary for lim-slip diffs, not for some synchromesh trans (apparently not this one). Checked out specifics for Redline MT90 (thanks Rob500) and found straight gl4 compatiblility.
Now have drained all Castrol Mineral HypoyC and refilled with Sta-Lube (mineral) Hypoid gl4 as a test and to flush out all gl5.
Will take out for a test drive and see where this gets us.

 

Just another hint...
Transmission fluids take some time to get into all the components and gears.&nbsp If you do not feel an immediate change in the transmission, give it at least a week to let the new fluid work into all the pieces...


-Ted

 

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 more time for lube to work in and then change to Redline MT90. deadRX7Conv's lists of links are EXCELLENT. If anybody wants to learn everything about synths, these are the links to check out. Thanks big time!!! It seems to be on the right track and the MT90 should complete the job.