Howdy folks!

Long story short, I am looking for specific and up-to-date recommendations for gear oil in an original 3-speed inside my 48 Stylemaster. I have the manual which recommends 1& 1/2 Pints of 90W "Transmission or mineral oil lubricant, or all-purpose/universal gear lubricant."

The only thing I have been finding online is decade-old conversations about how GL-5 is bad for the brass, and that GL-4 is what I want. The problem is, I am not finding GL-4 really anywhere. I have been calling around and discussing with some of the local folks I know who have knowledge of these sorts of cars and they are mainly recommending things like this...

https://www.napaonline.com/en/p/RPO01300?impressionRank=6

which do indeed have very specific information about how they are a substitute for GL-4 and GL-5 (Strangely) and are safe for yellow metals like brass.

Thoughts?


Side questions:
Pulling the plug off the rear-diff and transmission (filler plug), at what level should the oil be when cold? I have been told an inch below the bottom of the plug for both (Again, transmission filler plug). Sound about right?

Today's GL-5 oils do not have the high sulfur levels that would damage brass synchronizers. You are correct that it is difficult to find GL-4 only oils these days.

Any 85w-140 gear oil will work well.

There is lots of guidance about the "best" oil level. Some say 1" below the fill hole, some say 1/2" below, and some say you should just be able to touch fluid with the tip of your little finger when you stick it into the fill hole and bend it.

I typically fill until it is just ready to run out.

I have used GL 5 in my cars for the last 50 years with no problems.

I do not worry about the GL part of the oil but use the 85/140 oil and fill to just before it spills out of the fill plug and top up when your little finger doesnt reach oil. The same applies to the differential.
Tony