Dear Car Talk:

I have a question about using tractor hydraulic fluid in place of Dextron transmission fluid in my car. Do you have any opinion on whether it is a suitable substitute? Better? Worse? I have heard valid arguments in both directions.

-- Marc

I'm glad to see you're a guy with time on your hands, Marc. Not everybody has time to dream up such creative ways to void his warranty.

I've never used tractor hydraulic fluid in a transmission. I did use it once in a salad, but that was by accident.

Here are my thoughts on why it's not worth the risk: About four or five decades of research and development have gone into perfecting the molecular design of Dextron transmission fluid, which GM uses and recommends for all of its cars. They've gone from the original Dextron, back in the 1960s, which used sperm whale oil as a lubricant (no joke), to the current-generation fluid, called Dextron VI. It's formulated at what GM engineers consider just the right viscosity to operate the transmission correctly, control for heat, optimize fuel economy and lubricate the transmission parts to protect them from damage.

And they're the folks who have to warranty your transmission and pay for its replacement if something fails. So I'd lean toward their recommendations.

I'm not saying tractor fluid wouldn't work -- it is hydraulic fluid. But why would you risk it ... unless you were able to steal the tractor fluid from work? Seriously, you've got a $3,000 transmission in your car, Marc. Adding 10 quarts of Dextron costs about 30 bucks. And you're only going to change the fluid once or twice during the life of the car.

So you're not saving money.

I'm guessing you talked to some guy at a bar who said, "If it's designed for a tractor, it's got to be really tough ... more than good enough for a car. The heavier-duty, the better."

But that approach is not necessarily true. For example, if you put extra-heavy-duty shocks on your car, they might last longer than standard shocks. But the savings wouldn't outweigh the costs of treating the concussions you got from bouncing off the roof every time you hit a bump.

If you decide to ignore my advice and give it a shot, let me know how it turns out. I'd be curious, in the interests of science. But if it's my car, I'd stick with the manufacturer's recommendation, and that's what I'd suggest you do, Marc.