Summershandy

Recommended differential oil

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Need some thoughts on the grade of oil for the rear axle of my '54 Pontiac Star Chief. Manual recommends SAE 90. I have the following available:

 

80W90 

75W90 synthetic

75W140 synthetic

 

Reason I ask is I'm not familiar with what 64 year old gears would like to bath in these days!

 

Thanks

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75W/140 synthetic. Your OLD gears will breathe a sigh of relief. Hypoid gears like to take their "bath" in the best lubrication possible. The heavier oil at high operating temperatures will also leak less (or not at all), if you have any leaking or weaping now. Synthetic oil is more resistant to shearing down to a thinner oil too. I once had a leaking pinion seal on a '73 Cad. I often used it as a high speed, long distance freeway flyer. TOTALLY stopped with the heavier (at operating temperatures), oil. There are VERY few cases where the very best lubrication available will not provide benefits. Engine oil has been, and will be discussed here forever, so I will hold my fire. Synthetic grease should be MANDATORY  !!! Many years ago I was in Billings, MT needing to pick up some Mobil 1. The Mobil gas station directed me to the feed store, as this was long before the general public had any awareness of synthetic lubricants and the benefits thereof. But the operators of heavy agricultural equipment already knew. I got into a discussion with some of the customers at the feed store when they saw this stranger grabbing the Mobil 1. They informed me that wear at greased articulation points ceased to be an issue once they changed to synthetic. By the way  : Good choice of words in your question. I often think of what my machinery would "like" !  I hope you get some great runs this season !    -  Carl 

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2 minutes ago, C Carl said:

Engine oil has been, and will be discussed here forever, so I will hold my fire.

Understood. I don't wish to start a heavy debate but merely to ask others what they are running along with any opinions and thoughts. All comments are greatly appreciated.

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Make sure whichever oil you use is for differentials. This should be marked on the container. GL-5  85W-140 rated oil is what I have used. JWL

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When the manual says SAE 90, I would not go past that viscosity. An 80W-90 is the low half of the 90 spec, and 85W-90 is the high end of the 90 category.  Both are single grade 90 oils, with defined cold viscosities. Some people go higher to compensate for gear wear, and end up with more wear on the closer parts.

 

You can look up spec sheets on line and graph the oils you are considering with this handy tool (by default, it starts with some typical motor oils): 

Widman Graphing tool

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Esteemed Richard Widman : I am reminded of Oscar Peterson's observation when informed that Art Tatum was in attendance at one of his performances. "God is in the house" he told the assembled. Please clarify. If you were to choose between a straight 90 conventional EP gear oil , and Amsoil 75W/140 Severe Gear Synthetic , which and why , please. Many people here consider themselves your disciples. I have another automotive question , but one does not impose on God lightly. Reserving the automotive question , may I simply ask at this point : In the Heaven-on-Earth which is Cochabamba , do you know Freddy Burgoa. ?              Profound thanks on behalf of all of us motorheads.     Among your disciples ,     -   Cadillac Carl.  (Unoficial speed record holder , Arequipa - Puno  , and Puno - Desaguadero  ,  1978) 

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My 40 Buick owners manual says to use 80/90. I use gl-4 as opposed to gl-5.

Gl-5 may not be good for soft metal such as brass.

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And there is brass in your diff? I doubt it. Maybe in the gearbox though if it has brass baulking ring synchronisers and the bushes inside the countershaft gear.

 

GL-5 is not necessarily bad for copper and copper alloys. The Copper Strip Corrosion Test ASTM D130 is what you want to see the results of. 1a for 40 oC and 1a (or 1b) for 100 oC.

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