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If you’ve ever had to change the rear differential fluid on a late-model GM truck, car, or SUV, you’ve probably had to buy some of this 75W-90 diff fluid made by AC Delco, but have you ever wondered why it smells like grapes?

Adding scents to automotive fluids, while possible, is just plain unnecessary since car enthusiasts and people who actually work on cars for a living are used to such smells with some fluids, like a whiff of gasoline or a dollop of motor oil, are arguably pleasant. So, why does GM’s ACDelco lineup of rear differential 75W-90 synthetic fluid/lubrication smell so much like grapes? Well, the simple answer is because it actually DOES smell like grapes and is even advertised as such on the actual product. Check out a closeup of said ACDelco lubrication below.

Since when has scented gear oil been a thing?

I have reached out to GM on an official story on why it smells the way it does but in the meantime, I’ve come across at least two origin stories.

The first one involves line workers in GM factories in the late ’90s and early ’00s that, when they were queried on improvements for their work, mentioned the awful smell of rear diff fluid they had to deal with, day in and day out. So, to better improve the working conditions of their employees, GM did them a solid, partnered with a chemical laboratory, and put in a blend of grape smelling esters to mask the putrid scent.

The other prevailing theory is that some customers returning to their cars after receiving some routine maintenance on their cars, including a rear differential service, complained about an awful smell upon the first ingress into their driver’s seat. To ameliorate their concern and at least deal with the source of the issue, GM added the grape smells into the diff fluid.

Since GM added some of the good-smelling stuff to their diff fluids, a whole host of diff fluid manufacturers have funnily enough done the same, even choosing grape as their scent of choice. Royal Purple and Mobil are two notable ones.

Diff fluid smells bad, it’s just the way it is. My guess is that since it’s so viscous at room temps, plus all the modifiers added in, thick diff fluid scent tends to hang around more than thinner motor oil.

Although I never had to deal with rear diff fluid, I’m glad that GM cared enough to change their formula.

Have you ever used this “grape juice” in your GM cars? Do you have your own theory of why it smells the way it does? Let me know in the comments below!

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