1989-1993 Dodge 5.9L Cummins Differential Fluid

Differential fluid for the 1989-1993 RAM 5.9L Cummins

First-gen Cummins trucks put a Dana 70 under the back of every truck and a Dana 60 under the front of 4WD W250 and W350 models. The rear is the axle with a published spec: SAE 75W-140 synthetic. Full-float axles that go too long on old fluid tend to whine and run hot when towing, so a fluid change is a smart first job on any first-gen that is new to you.

What we carry

For the rear Dana 70: Lucas Oil 10121 Synthetic 75W-140 and Royal Purple 01301 Max-Gear 75W-140 synthetic. Also in this collection: Hot Shot's Secret Blue Diamond HD synthetic and ADRENALINE synthetic racing gear oils in 75W-90, 80W-110 and 85W-140, and Lucas Oil Synthetic 75W-90 plus Heavy Duty 80W-90 and 85W-140.

Choosing a grade

For a stock rear axle, stick with the 75W-140 synthetic spec and pick the brand you trust. If the truck has a rebuilt or swapped axle, follow what the axle builder recommends. For the front Dana 60, check your service manual rather than assuming it matches the rear. The other grades here are for axles whose spec calls for them, not substitutes for 75W-140 in a stock first-gen rear.

Frequently asked questions

Is 75W-140 the right gear oil for a first-gen Dana 70?

Yes. The maintenance schedule for 1989-1993 5.9L Cummins trucks calls for SAE 75W-140 synthetic in the rear. Lucas Oil 10121 and Royal Purple 01301 Max-Gear are both 75W-140 synthetics.

What are the signs my differential fluid is overdue?

On these full-float axles, whine and heat under towing are the signs of neglected fluid. If you don't know when it was last changed, change it and look at the gears while the cover is off.

Can I use the same oil in the front axle?

The 75W-140 synthetic spec is for the rear Dana 70. Check your service manual for the front Dana 60 on 4WD W250 and W350 trucks before you fill it, and call us if you want a second opinion on the grade.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.