Well, all I can say is go wet your driveway with a garden hose. Then go park your wheeling vehicle there. Now spray the underside. If you don't see many, if any, rainbows then apparently not.
The average wheeling vehicle is taken apart all the time and usually has very limited amounts of petroleum products on them.
Next, it'd take a lot to do any damage. If we're talking a leak that loses a quart of whatever every 3,000 miles we're talking virtually nothing in the grand scheme of things. Mother nature can take care of a lot of such abuse if kept in small amounts. Is a stream going to be happy if 1,000 vehicles dripping a quart every 3,000 miles crosses it in one day? Nah. Is it going to survive thirty or forty vehicles with only half a dozen vehicles leaking anything? You betcha. It'll never know. The clown upstream that dumps his sewage or his laundry water next to the stream is causing far more pollution than you can imagine.
The streams are more polluted by the storm drains on our very own roads. Tractor-trailers alone create more pollution than the cars that traverse the same roads. And I'm only talking about non-air pollution.
If you ever noticed, you'll see that if it hasn't rained for 3-4 days and then it rains the road is VERY slippery for about the first half an hour. That's from pollution that just falls off vehicles.
If you want to talk about trail use, that's a whole new can of worms. Do offroaders tear up the land? Yeah. Does it make that much of an impact? Nope. There is virtually no difference between a 10 mile ORV trail and a 10 mile hiking trail. While the ORV people have a habit of making a mess of the ground the hikers do as well. Hikers like to tear the crap out of foliage over broad areas. They have a habit of packing out more than they pack in no matter what anyone says. The only difference is you see two ruts going through the woods instead of 3-4 footpaths.
Keep in mind that there are assholes in all forms of recreation. There will always be people getting blamed for other people's stupidity.
I suggest instead of people pissing and moaning about damage being done that they clean up public forests. Any and all common use of forest lands eventually returns to its natural state. Little impact is done by any form of use (whether on a bike, on a horse, or in a truck) in the grand scheme of things. However, trash and other solids are a real problem.
I don't see any Envirotards or bicycling or any other kind of non-hunting non-offroading club on Michigan's Adopt-a-Forest website.
http://www.cleanforests.org/volunteers.shtml