Well, even if you don't lube them, at least loosen the diffs. For whatever reason, most of them are way too tight. And that does put a load on the motor.
I think you had a poor connection. That will fry a motor, too.
The Neo magnets are here (in the US):
http://mini-zracer.com/shop/product.php?pid=441
I ran mine with the Neo magnets and full bearings (motor, too) this weekend. I used two lithiums (8+ volts at the start) - and the motor barely got warm! I ran it as hard as I could outside. But I do keep the diffs loose - just enough to slip somtimes from a dead stop upon full throttle.
New problem with the Neo magnets - it picks up all kinds of fragments from the ground! They are so strong that the whole Mini-x with batteries will stick to the side of my truck and stay there!
As far as I know, you keep using the 8T pinion with any of the three diffs.