Friends' Identical Cars Set the Stage for a Perfect Experiment

Dear Car Talk:

My friend, Greg, and I coincidentally bought the same make, model and color of car and in the same year! It's a 2WD 2010 Lexus RX-350.

We are both careful to follow the recommended maintenance specified in the owner's manual. For normal driving, the manual does not list changing the differential fluid as routine maintenance. It's recommended only if towing or driving in excessively dirty or dusty conditions. There is no dipstick, and changing the fluid is difficult.

Greg recently changed his, and the fluid was dark like prune juice (yum). Would you recommend it be changed, and, if so, how often? -- Jeff

Well, I'll give you two answers, Jeff.

From the point of view of a mechanic, it certainly can't hurt to change the differential fluid. This car uses a synthetic, long-life differential fluid. But the owner's manual indicates that under heavy-duty use (like towing, high temperatures), when the fluid gets hotter, changing the fluid is beneficial. After all, excessive heat is what breaks down every lubricant.

So you might not need to change it, but it's clear that no harm will come from replacing it. And possibly some benefit -- especially on an older, high-mileage car.

And by the way, if your pal Greg checked his brand-new differential fluid after driving for two weeks, it would also look like prune juice. That's just how used differential fluid looks -- it's disgusting -- even after minimal use.

My second answer for you is from the point of view of a scientist, Jeff. You have set the stage for a perfect experiment. Same car, same age, same maintenance program -- except for the differential fluid change.

The scientist in me would tell you -- by all means -- don't change your fluid. Instead, take the $150 and treat your spouse to a nice dinner out. And then keep driving and see what happens.

Let me know which runs longer, your differential or his marriage.


When it’s time to say goodbye to your old car, here’s the perfect solution: Donate it to your favorite NPR station. They’ll even pick it up at your convenience. Here's how.

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What the stars say about your car for 7/6/2022
Do not despair. Money can fix some of your upcoming problems. But it will be at least three times the number you have in mind.
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