These are toy cars, not F1 cars. I think many of your are overthinking it.
"thumbsup"
You only need a light coat of something on your trans gears. Just enough to reduce friction. Shock oil, diff oil, gear oil, motor oil, silicone spray, pretty much anything that is slick and will stick to the gears. Hell, you could blow a booger on them and it would work, at least up until the point it dried up. Like CHUD said, you'll break them before the chemicals attack the gears and cases enough to compromise them.