I need to change my differential oil and I can't find anywhere what type of oil to use. Chilton book doesn't say and neither does the manual. 1993 F350 7.3 IDI, RWD, C5 axle code, 4.10 ratio. Anybody know?

 

Well that's a limited-slip diff. The '93 owner's manual has nothing about the diff in the Fluids section? Odd. I forgot to check last night, bit IIRC either 80W90 or 75W140 should suffice, if you add friction modifier. Any synthetic gear oil will also work, without the need for friction modifier.

 

i like mobil 75-90 synthetic ..

you will get better mpg with synth fluid in the chunk ..

i don't know if that's the best for a posi though ..

 

I used Red Line synthetic. At least I think it was called Red Line. it was a long time back.

 

I have the Sterling open rear diff, same advice for me?

 

redline is also good .. but mobil seems more available ..

either one is good ..

75-90 synth / best

or 80 - 90 gear oil

 

Quote:

Originally Posted by Impreza_gc8

I have the Sterling open rear diff, same advice for me?

Sterling 10.25, 10.50 Fluid Spec: 75W-140 gear oil
Sterling 10.25, 10.50 Fluid Capacity: 3.25- 3.50 quarts
Additive for Limited Slip Differentials: 4 ounces, Ford XL-3 Friction Modifier

Source:
Sterling 10.25/10.50" Differential Fluid
How to Replace the Oil in a Sterling 10.25/10.50" Rear Axle
Sterling 10.25" & 10.50" Differential Service

More information on the Ford "Sterling" 10.25/10.5 rear axles:

Ford 10.25 & 10.50-Inch Axle Tips - Axle Examination

If the gear lube has "LS" designation on the label then you don't need to add the friction modifier - it's already in the product.

Just one example:
Mobil 1™ Syn Gear Lube LS 75W-140

Full description:
https://www.mobil.com/English-US/Pas...Lube-LS-75W140

When I change mine I'll be using a full synthetic, like LS 75W140 but I don't know what band yet.
I'm choosing synthetic for better performance and durability/longevity across a wider temperature range. (a typical advantage of synthetic oils is withstanding higher temperatures before breakdown of the lubricant). I'm not certain but I think that synthetics also flow better than non-synthetic in extreme cold temperatures

By the way, I would recommend checking the metal tag attached to the differential to confirm which rear end you actually have, it's ratio etc., because it may have been changed and not match the Axle code on the Specifications sticker or in the VIN (Vehicle Identification Number).