What oil to use?

[Mayhim]

If you are limited by weight, then you still have to consider brand. Not all 5w-30s are the same viscosity.

The two thinnest 5w-30s that are easy to find are Havoline Deposit Shield and Castrol Syntec. The thickest 5w-20s are Castrol Syntec and Castrol GTX.

[unstable bob]

As far as your acetone question goes, the most common formula I've come across is 3 oz acetone per 10 gal of gas. And you should be fine if you want to try it in your Saturn w/ the plastic lines. That is a very common style of fuel delivery system...my Cavalier has it, and I've run boo-coo amounts of acetone through it w/o any probs. In fact, I've used an acetone/windshield washer fluid mix in an H20 injection set up that has plastic lines and a plastic reservoir w/o any probs.