Alex,
Got to WalMart, ask for fishing reel grease. Works fine lasts a long time. Nothing wrong with lithium but you just don't need a gallon of the stuff, right? I'd also think about swapping the prop for one with a bit finer pitch, or slightly smaller diameter (cavitation). Give it a try first, though! May not need to worry about it. Brushless motors are high current draw thingys. Don't expect hours of run time. And since your ESC isn't quite heavy enough for the max current, you might take it a bit easy, keep your foot off the gas and check for heat fairly often. Too much is always better than not enough, just not as 'frugal' in the long run...
Good luck.
- 'Doc