Never really thought about there being more than one type of 90 W gear lube. Always used the same stuff in transmissions and rear ends in the past. My understanding is that GL5 is for rear ends and GL4 is for manual transmissions like my toploader. I know the modern 5 and 6 speeds use ATF or some similar oil, not 90W.  There is some question as to how well Synthetic will work. Synthetic is rated GL5. I can find non synthetic rated GL5 , GL4. Also I see both 75w-90 and 80w-140 recommended. What are you using and why? I have a quantity of 80w-140 rated GL5, GL4 on hand. Transmission has been rebuilt. Plan to change the oil after 500 miles or so. Will GL5 cause problems in the transmission?   While on the subject, is Synthetic GL5 what I want in my 9" rear end? I know I need the Ford additive if I have Traction Loc.  

Regarding the rear end, you probably do not need any additive nowadays as nearly every gear oil there is already has it.  Finding a gear oil without it is very difficult.



Lawrenceville, GA

Any type of hypoid gear lube is not recommended for transmissions that have yellow metal parts, i.e.: brass synchronizers, etc. The hypoid additive attacks the brass parts under heat and pressure, causing premature wear.

NAPA has a mineral based gear oil for transmissions with yellow metal parts. I think it has a GL-1 rating. I'd have to go out to the shop and look. I put it in my T85. It was fairly reasonable on price also.

Durham Missouri

I understand the potential problem with GL5 in transmissions. My problem is finding something only rated GL4. The question is will something rated GL5 and GL4 cause problems with the brass syncro's?

As long as it's non-hypoid oil you should be OK. I don't know if that exists in a GL4/GL5 rating, though. Is the trans original to the vehicle or a swap meet/salvage yard treasure? If it's original or you know what it came out of the shop manual will have the specified gear lube listed. If you don't have a shop manual someone here probably does and can look it up.

http://www.davidkeetoploaders.com/specifications.htm
Here you go.

Durham Missouri

That's the first place I looked. He doesn't address GL4 or GL5. It's a Ford Toploader. I know a lot of you are running Toploaders.

Just to add to, or possibly clear up, some of the confusion. I learned a lot...

http://www.widman.biz/uploads/Transaxle_oil.pdf

StaLube has a GL4 that is a hypoid but claims it is formulated not to hurt yellow metals. Thats what I have been using.