I use common houshold oil (e.g. 3 in 1). I've tried spokeprep and didn't like it. I still have an *almost* new bottle if anyone wants a discount. It's expensive and unnecessary.
Fisherman, I'm not sure how heavy oil will avoid spoke windup. Spoke windup is something you're likely to deal with no matter what. The only thing I can note from my experience is that straight gauge will windup less than swagged, and thicker swagged will windup less than thinner gauge swagged. ...But they will all windup.
When starting out on your first few wheels, you can use a piece of tape on the spoke as an indicator. As you tighten nipples on the tensioned wheel, you'll see the tape rotate as an accurate indicator of how much the respective spoke you're tweaking is being twisted.
Good luck!