Lark327

My local stores don't have any GL-4 gear oil, but they do carry 90 weight marine oil which is GL-4. It's formulated for boat drives and has an additive that protects the integrity of the oil if a little water gets in it which is likely on a stern drive. Is there any reason you couldn't use this in a Muncie?

20mercury

Good question!

I wonder if you could email Mercury Marine and ask?

resdoggie

Why??? Use GL5 that's formulated for safe use with yellow metal.

sullyman56

I bought some Redline MT-90 synthetic gear oil and it was delivered to my house. It’s GL4 and I didn’t have to go hunt for it at a store.

rberman999

Napa has Sta-Lube 85W 90 gear oil, GL4. Order on line pick up in store. Many rebuilders state DO Not USE GL5, as it breaks down the brass / bronze synchros. Just passing on what I found for source and info. I read many people use GL5 but almost everyone I read who rebuilds transmissions said No GL5. Good luck, Russ

resdoggie

Those builders have their heads in the sand. That "problem" was sorted out years ago. But some folks hold onto the myth and keep spreading it.

rberman999

I guess you could be right but a quick search turned this up:


https://www.machinerylubrication.com...itives-effects

Worth a read , short article implies that active sulphuric additives VS non-active sulphur can have a big effect and cause attack on yellow metals. So as long as you know the additive in your GL5 lube your fine. I'll just stick to GL4. Good luck, Russ

resdoggie

And that is why most GL5 lubes are safe to use these days.

rberman999

One more quick reference from a lube manufacturer that again states use GL4 when called for, this is from 2019, using GL5 can cause damage. It's important that everyone does what they think is right for their transmission. As always, good luck, Russ.


https://www.rymax-lubricants.com/blo...gl-4-and-gl-5/

rberman999

A good article that gives the proper standard that your GL5 must specify to be safe as a GL4 substitution. In addition to the GL5 it must also specify that it meets GL4/MT-1. If not, don't use GL5 gear lube. I must admit they say it is best to use conventional GL4 lube. This article also outlines how to change the gear lube and how much you will need. It's a good article from 2015. Good luck, Russ


https://oldcarmemories.com/muncie-bo...on-oil-change/