Help with manual trans fluid selection please...

[limey66]

A quick panic – this weekend is one of the few I get to spend working on the Mustang, and I wanted to change the transmission fluid, as I don’t know it’s history. The car is a 1966 with 289 V8 and three speed MANUAL trans.

What fluid should I use, I remember seeing somewhere that a hypoid oil shouldn’t be used, is that correct?

Being in the UK, it’s difficult to identify the correct grades of oil required, I asked Castrol and now I’ve checked, I see they’ve told me to use Auto trans fluid. Is that correct?

 

[65ShelbyClone]

What fluid should I use, I remember seeing somewhere that a hypoid oil shouldn’t be used, is that correct?

Being in the UK, it’s difficult to identify the correct grades of oil required, I asked Castrol and now I’ve checked, I see they’ve told me to use Auto trans fluid. Is that correct?

No and no. You need hypoid gear oil like what goes in your differential. I dont know what grade, but the wrong lube is better than no lube. Modern manual transmissions use ATF, btw.

 

[10secgoal]

Yes, modern ATF only applies for late model trannies with much tighter tolerances than what came in our early cars.

 

[D.Hearne]

Use 85W-140 Gear Oil, or something similar in it. Certainly nothing thinner than 50 weight oil. The warmer the climate, the heavier it needs to be.

 

[66HertzClone]

You need an 80w or 90w GL4 gear oil. The GL4 being very important! Gear lubes rated as GL5 contain an additive to prevent wear under heavy loads, EP (extreme pressure) as it is known. This additive contains sulfur, which will attack with vigor yellow metals. What's the problem with that you ask? Think of those nice syncros in your trans being eaten up by this EP additive.

GL4 lubes are available but just not as easy to find as the GL5 lubes, good luck, Jeff

 

[D.Hearne]

I've never paid attention to the GL-4, GL-5 thing and have never had problems with eaten up brass syncros. If what you say is true, then the GL-5 must be more plentiful than you think, or I just got lucky for the last twenty + years.

 

[limey66]

Nuts.

I just bought gallons of 80 weight EP oil for the trans and rear axle, now I'm not sure whether to use it in the trans or not.

I'll check the level, and if it looks OK, I might just leave it for the time being.

I like D.Hearne's approach, if it's OK, then I'm happy. If it damages it; heck, it's only a three speed and a busted tranny will mean I can justify a T5 swap...

Anyone in the UK want to buy some EP gear oil?

 

[66HertzClone]

I've never paid attention to the GL-4, GL-5 thing and have never had problems with eaten up brass syncros. If what you say is true, then the GL-5 must be more plentiful than you think, or I just got lucky for the last twenty + years.

GL5 is the easy one to find, all differentials need the EP additive. Since most modern manual transmissions use lighter lubricants like ATF or even in some vehicles 5w30 oil, the use of GL4 gear lubes have dropped making more difficult to find, especially in quart sized containers.

 

[limey66]

I've found a copy of the 'shop manual, and that calls for a 'mild' EP gear oil, so I'm going with the Exxon Valdiz load of EP gear oil that I have.

Thanks all for your comments and guidance.