Mostly Mechanical

Auto & Truck Oils, Lubes & Filters – Separating Technology from Hype

Differential Fluids, Differential Covers, and Towing.

OK, what’s the real scoop on differentials?  What do aftermarket differential covers do for you?  Should you buy one or make it yourself?  When should you worry about it?  When do you need a temperature gauge for your differential?  Those questions and more came up in a recent online user forum, and the experts’ answers were excellent.  If you do any towing, I believe this information is critical for you.

OEM’s agree that to maximize your differential life you need to do your first fluid change at 5,000 miles, and lubrication and drivetrain engineers will add that a high-performance synthetic is the best and longest-lasting choice.  Maybe you’ve heard that, but what synthetic should you choose?  Remarkably, there are downright embarrassing differences in the tested performance of gear lubes on the market.   In fact, using the wrong one in a towing application will probably take your differential into early failure.   You can download a free research study detailing the performance testing of 14 name-brand gear lubes.  Think it doesn’t matter much?  On the contrary, we found it very disturbing that over half of the name-brand gear lubes failed one or more of the standard performance tests.

That study is also excellent because at the beginning, as background, it outlines the results of operating-temperature studies done on differentials in towing applications.  The information from those studies is eye-opening.  So enjoy.  And remember, your entire vehicle and towing load rests on TWO GEAR TEETH in your differential: your gear lube choice is critical !


February 25, 2008 - Posted by | Amsoil, Diesel, Diesels, Fleet, Lubrication Oils & Fluids, Vehicle Maintenance | , , , , , ,


  1. looking for a polished differential cover for my 2003 chevy 2500hd silverado and the best fluid for the diffthanks jerry.

    Comment by jerry cootware | June 4, 2008 | Reply

  2. Jerry –
    There are several diff cover manufacturers out there, and “polished” means that visual appearance is important to you, right? Only you can determine what “look” you want the rear of your truck to have, and whether you want it fully polished, or partly polished and part rough or painted. For example, I personally like the look of the ATS diff covers. The best suggestion I have is to get a copy or two of Diesel Power magazine, and look through the ads to see what you like.

    Also, some design features to consider in diff covers are the drain plug, fill plug, checking fluid level, and whether you want the option of installing a temperature sensor.

    The best fluid? Synthetic. Best brand? Amsoil – hands down. That’s what the industry-standard-test data shows (see link in article above), and OEM engineering insiders know that nearly every successful pro racing team runs Amsoil gear lube in their diffs. Amsoil invented and has dominated synthetic gear lube, and few companies have seriously attempted to compete with their performance or reasonable pricing.

    The best Amsoil gear lube? That all depends on your use of your truck. The three guiding factors are the level of performance modifications you’ve done (engine torque/HP), the importance of fuel economy, and the level of severity of service: towing an occasional jet ski trailer, vs commercial 12,000 lb trailer hauling, vs rock crawling, etc.

    The more power, and the more demanding the application, the heavier the fluid weight you should use. For mostly highway miles with a 5,000 lb trailer or less, 75W-110 is Amsoil’s product code SVT that offers a great balance of protection and fuel economy. The long general-application towing and racing favorite is 75W-140 (Amsoil product code SVO). Finally, the SAE 190 and SAE 250 (Amsoil product codes SRN and SRT) are great choices for high-power severe racing and off-roading conditions where a small loss in fuel economy and wheel horsepower is not as important as avoiding gear failure.

    We’ve been talking about the rear differential, but 4wd trucks also have a front diff. In most cases, Amsoil’s 75W-90 (SVG) or 75W-110 (SVT) are the correct choice for the FRONT differential. But high-power launches in 4wd will probably want to stick with the 75W-110 or 75W-140 in the front diff.

    Comment by autoengineer | June 4, 2008 | Reply

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