1940 buick transmission gear oil

[lhend50]

I have seen people suggest an 85/145 gear oil. Also GL4. I have looked for a GL4 oil with that ratio and have not found it. I can find royal purple in 75/145 but it is GL4 and GL5 rated. Is this usable. If not, the only GL4 I find has a weight no greater than 90. Any suggestion would be appreciated. Trans has been completely rebuilt. 

 

[Stude Light]

GL4 or GL1 90W are both fine choices too. I use GL1 90 weight in my 1939 LaSalle as it shifts better with that vs a heavier oil. With synchronizers you need to get the oil out of the grooved synchro rings so they “lock on” to the cones to get the gears to speed match and the heavier oils resist squeezing out. You don’t need the GL4 EP friction modifiers required for the high contact stresses and sliding friction found in a hypoid gear in a transmission. 

Edited July 16, 2023 by Stude Light (see edit history)

[lhend50]

10 minutes ago, Stude Light said:

GL4 or GL1 90W are both fine choices too. 

Thanks, that I can find. 

[Bloo]

GL4 and GL5 are ratings relate to suitability for use with hypoid gears (as used in many rear axles) and have nothing to do with a transmission. Technically GL5 is heavier duty hypoid oil, and anything that is GL5 probably also meets the specs for GL4 but not vice versa. Shopping by GL number for a transmission used to work a long time ago, but that was only a happy accident and in 2023 it is the road to madness.

 

Transmission synchronizers are brakes and have needs that are completely at odds with the needs of hypoid gears. There are dedicated Synchromesh oils these days, usually synthetic. I like Redline 75W140NS (note the NS, it is important), but there are many others. If you aren't going to use a dedicated Synchromesh oil, I would go for the GL-1 @Stude Light suggested. GL1 just means that it is mineral oil without a lot of additives. That or motor oil would probably shift better than something with a bunch of hypoid gear additives in it.

 

[EmTee]

I am using this in my '38 Century's transmission:

 

https://www.oreillyauto.com/detail/c/masterpro-chemicals/oil--chemicals---fluids/grease---lube/gear-oil---additives/140w/754ba5cbea99/masterpro-chemicals-conventional-gear-oil/mp03/80040

[lhend50]

11 hours ago, EmTee said:

I am using this in my '38 Century's transmission:

 

https://www.oreillyauto.com/detail/c/masterpro-chemicals/oil--chemicals---fluids/grease---lube/gear-oil---additives/140w/754ba5cbea99/masterpro-chemicals-conventional-gear-oil/mp03/80040

Thanks, I actually found GL! 90 at NAPA. Not on the shelf, but they had it upstairs. 

[Bhigdog]

FWIW.........I topped off my 39 chevy trans with 85/145 because that's what i had on hand. Very noticeable difference in the shifting. Synchronizers are not working so nice. I'm going to drain it and go back to 90W..............................