Oh No Another Oil Thread - MT90 GL4 and 75W90 GL5

I'm replacing all the fluids in my 161,000 4x4 manual "new to me" Tacoma. The manual says 75W90 GL4 or GL5 for tranny and transfer case, but ONLY GL5 for the differentials.

Therefore, I could easily just get Redline 75W90 GL5 Gear Oil for everything and call it a day.

HOWEVER...

Some people have reported better shifting out of their manual transmission with Redline MT90 GL4. (This from doing my homework around the forum).

So here's what I want to know:

Should I keep it easy and do Redline 75W90 GL5 gear oil for them all OR do 75W90 GL5 for the transfer case/diffs and MT90 just for the tranny? Redline themselves recommend MT90 for the tranny.

 

By the way, another option would be to use the non-limited slip version (NS) Redline 75W90NS (58304) everywhere. It's still GL5, but might be a sweet spot in the transmission in case the regular (limited slip) Redline stuff is too slippery for the syncros. This might fit right in between using MT90 GL4 for the tranny and be better for both the transfer case and manual tranny.

 

To me everything get the same gear oil.

It is just easier for me using the pump in the 5 gallon pail.

Then I just use the local brand gear oils.

Bottom line it is your Truck.

 

Right. I'm just trying to gauge feedback from experiences here. If MT90 GL4 is going to make a difference in improved shifting over using 75W90 in the tranny, then I don't mind having to use two different kinds of oils. After all, gear oil is not something you change as often as engine oil.

 

Anyone else have any input on this? I was wondering the same thing

 

I just did my entire truck when I bought it a couple of months ago.

I put gl5

$30 75w-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75w-90 in transfer case and front and rear diff's. Found some gl4 75w-90 for transmission.

I notice 0 difference than with the old fluid I drained out.

I asked this same question when I did mine.
One forum owner stated he used gl5 in his manual trans for a lot of years with no ill effects.

 

Good to know, thanks. Sounds like these are are pretty resilient. I guess that the strict use of GL5 is more for limited slip diffs and the strict use of GL4 in transmissions is also for those that are well-known to be notchy and temperamental.

However, with all the research I've done so far might as well do it right. Since I'm going to be using Motul, and Motul 300 75w90 is a GL4/GL5 oil, I'm just going to put that in both diffs, transfer case and transmission. They also have a 300 75W90 GL5 LS (Limited Slip) which is of course something our USDM trucks don't have. I'll pick up a bottle of that for my Miata's rear LS diff.

I'm not sure how and oil can be both GL4 AND GL5, but there you have it. My guess is that an oil that's both GL4 and GL5 is good for components like these on your trucks that are happy with either.

 

The use of gl5 in our diffs has to do with the extreme pressure and heat properties for the ring and pinion. The reason for a gl4 on the transmission is due to the fact that a gl5 can be harmful to and brass component's that may be in there. However im not sure if we have any brass in our trannys. I used the mt90 gl4 in mine and noticed zero change from the ancient black stuff that was in there. When a gear oil is advertised as being gl4 or gl5 it is just labeled that way for dumb people that worry they cant use a gl5 oil in something that is rated as needed only a gl4. Its not really a combination so to speak of both ratings. For example an older rig might show to in the manual to use a gl4, but using a gl5 would be perfectly fine.

 

Get something rated both gl4 and gl5. That's really all you need to worry about.

 

MT90 helped with cold weather performance, and that's about the only gain I noticed.

I too have heard not to use GL5 in the transmission, but not sure the validity of this statement. Especially because it says you can use either in the manual.

 

Allegedly the higher sulfur content in GL5 compatible oils is bad for the the syncros in our transmissions. Who knows whether it actually makes a measurable difference.


That said, I just ordered MT-90 for my transmission because it's supposed to really make a difference in how the transmission shifts when cold.