Is GL5 gear oil really bad for our transmissions?

[John Smith]

Has anyone experienced worn synchros due to using GL5 gear oil in their 3 or 4 speed transmission? A gear head buddy of mine with a Willys called me up and asked me if I ever heard of problems with GL5 spec gear oil and older transmissions. I hadn't, but started searching the Web and found a bunch of information that claims GL5 can damage our old transmissions and we should be using GL4 spec. One story I found contained a picture of a LC tranny synchro supposedly killed by running GL5.

 

[desertpat]

I put GL-5 into a 9 year old Pajero with 90k & manual 5 speed tranny.
Instantly noticed a "stiff resistance" putting it into 1st gear.
Other gears were stiff also but less noticeable.
Changed oil back to GL-4 and the stiffness was less but still noticable .... even to 125k and when warm.
My oil guru (Phd) said it was "residue".
I believe it coats metal and should be used in hypoid gears as in differencials & without syncros>>>
cheers

 

[SteveH]

I've been running GL5 in my stock FJ40 tranny since 1990 - no issues. Not sure it's the end of the world.

 

[snobdds]

GL-5 is not good to "yellow metals" due to the high sulpher content. It eats away at them over time.

I am not sure if the 40 series trans have brass syncros in them or not, but in my 88's W56, GL-5 is not reccommend. The FSM specifically states to use GL-4. Now the confusion comes from companies that state their gear oil is both GL-4 and GL-5 compatible.

I have always used Redline MT-90 and never had a problem with the syncros.

 

[Ambrew]

The key thing is over time. If you can, use gl4 (I'm using redline mt90) but gl5 is better than nothing. Depends how long you want it to last without a rebuild. And yes, there is brass in the trans.

 

[Cruiserdrew]

It is my understanding that all modern GL5 oils are buffered to avoid the (minor) problems with brass syncros. GL4 is an obsolete spec anyway-try buying some other than at Redline.

I'll bet on IH8MUD 90% or more of the manual transmissions are running GL5 gear oil right now and it isn't exactly common that the syncros fail.

With that said, the Redline MT90 works really, really well in my H55f. My other transmissions-SM420 and H41, run GL5 no name (Coastal) gear oil and work fine.

Anyway, I don't think it really matters.

 

[thetoyotaman]

My '82 owners manual says to use GL-4 or 5 and I have GL-5 in it right now. However, I am going to switch out to GL-4 because it has made my other car shift better.

 

[DougAustinTx]

Amsoil also makes a true GL-4 gear oil. I drained the GL-5 out of my new H55 when I started reading of the problems with synchros and GL-5. To me, as long as a true GL-4 is available, (especially a synthetic version) why risk the loss of my synchros?

 

[bsevans]

I use Amsoil products in all my vehicles. They sell GL3, 4 & 5. They recommend the GL5 for the FJ40 Tranny and Xfer case. I have been running synthetic GL5 for more years than I can remember. I have not had any issues in the Tranny or Xfer case as far as gear or syncros go. The only issue I have is with the seals and that presented itself long before I switched to GL5. I also run the same gear oil in both axles.

 

[Diesel]

Ran GL-5 in my '84 BJ43 for 12 years, never an issue. Have it in my current '80/'83 BJ42 and all seems well.

 

[DougAustinTx]

I was talking to the tech guy at Richmond Gear today about the lube for my Richmond 6 speed (it's in a Camaro, not a 40) and asked hm about the GL-4 vs GL-5 issue. He told me that GL-5 will damage the brass synchros if it has the friction modifier additives, which most GL-5 do. I asked him about RedLine gear lube. He said the Redline GL-5 NS is ok to use because the NS stands for No Slip additive. As I mentioned earlier, Amsoil's regular GL-5 DOES have the friction modifier additives which apparently harm the synchros over time, but they also have a line of GL-4 gear lubes whch specifically do not have the additive.