Gear Oil Choices?

[Ghostultramarine]

Wanting to replace my potentially 50+ year old gear oil, the local shop did not have 90 weight oil in stock on the shelves.

Lots of synthetic gear oils with a range that includes 90 (ie 75w-x), etc.

Are these permissible to use in our differentials?

Mine is a 1968. And how much gear oil is needed to fill the differential after the olde stuff is sucked out? (I want to make sure I buy enough in one trip.)

 

[Trace 300 Hurst]

Mineral-based gear oil, and no trip to the shop: https://www.amazon.com/Valvoline-75W-90-High-Performance-Gear/dp/B000CQ4DJ6/ref=sr_1_4?crid=3UY6ZW3KHRO4E&dchild=1&keywords=gear+oil+75w-90&qid=1622816354&s=automotive&sprefix=gear+oil,stripbooks,171&sr=1-4&th=1

 

[Ghostultramarine]

Thank you for the info! That stuff at 75-90 would be perfect for where I live!

Is mineral based preferable over synthetic? If so, why?

 

[Trace 300 Hurst]

Is mineral based preferable over synthetic? If so, why?

Firstly, these diffs were designed to run mineral, and have done so for zillions of miles. I see no advantage in our low-perf, low-speed, low-load cars to use a syn that's probably *better*, but simply not necessary (which is why I prefer mineral 10W-40 in the engine plus a zinc additive...as there is NO need for Mobil 1, unlike my high-revving, highly loaded C8 Vette engine).

In my 'opinion', it probably doesn't much matter if you mix them a bit , but why bother worrying about it? Go with the mineral 75-90 and drive it for another 50 years.

 

[Trace 300 Hurst]

Do you have a suction gun? It's how you get the old stuff out and the new stuff in. Neither task is much fun, and you will be getting gear lube all over the damn place. BTW, fill the diff until it just dribbles out of the fill hole.

https://www.amazon.com/Arcan-Suctio...&s=automotive&sprefix=suction+,aps,189&sr=1-4

 

[CBODY67]

Normal plain jane 90W axle lube is just fine. Worked great when that was all that was around!

 

[413]

Don’t fill it until it is coming out, at that point it’s over full.

 

[Trace 300 Hurst]

Don’t fill it until it is coming out, at that point it’s over full.

Agreed. To keep from making a mess (because thick gear oil pumped in too fast and too much will take a few seconds until it starts to ooze out), my method has been to fill, check with a finger in the hole, fill some more, check, fill....until you strike oil just as it reaches the hole.

Funny....can you imagine putting your car in the air and checking the fluid level every six months? I'm overdue by 50 years.

 

[CBODY67]

Many old service station guys usually had a gauge made of something like welding rod, although a right angle (individual) Allen wrench would be used to stick the short, right angle bent, end into the hole to gauge how far below the hole the lube level was. Quick and easy and consistently accurate.

Enjoy!
CBODY67

 

[Ghostultramarine]

That states the factory fill lubricant is satisfactory to -30.

Which would mean the factory fill gear oil is sae75.

Unless it’s sae 75-90.

 

[Trace 300 Hurst]

That states the factory fill lubricant is satisfactory to -30.

I don't do much cruising around or attend many car shows when it's 30 below.

 

[CBODY67]

That states the factory fill lubricant is satisfactory to -30. Which would mean the factory fill gear oil is sae75.
Unless it’s sae 75-90.

I would suspect that unless the car was originally sold into a dealership where the ambient temp was colder than -30 degrees F consistently, the car should have had SAE 90 in it from the factory. Multi-weight rear axle lube oil came later!

 

[Ghostultramarine]

Would the factory know where each car was destined as they were filling each differential?

I would think that’s done as or before the frame is assembled (?).

It does say “each vehicle”, not just vehicles destined for cold climates.