You can run some automotive grease, but you were right on the money when you asked about high temp. Grease has a temp rating for liquifying to coat the bearings. If hi-temp is used, effectively the grease will merely cut a channel through, and will be bearing on race. White lithium is acceptable and other low temp greases will work fine, although won't last as long as some of the bike specific greases, (yes, these are made by big companies, but are formulated for specific application).

I think if you work hard, put more like $80 into the bike (cables, brake pads on top of tires and tubes) you'll have a bike worth $20 or $30.

However in the world of hobbies and learning curves, this is cheap, fun, educational entertainment. I don't understand my wife's dish collection, but I do know she get loves them, loves shopping for them and my life is in danger if I make fun of them. That and they are alot more expensive than bikes, and not nearly as fun to ride. In other words, your decision is the right one.

Good riding