As far as impurities are concerned, it's not the type of solvent that matters but the source you get it from. Solvents will not leave behind impurities if the solvents are pure.
I've always liked using butane to make oil. Since it boils at temperatures above zero (C), you can boil the pot in a solvent without adding any heat. I just wrote up how I use to do it a few days ago:
I don't have it anymore but a few years ago I made a small device from six inches of about 5/8" i.d. steel pipe with threaded ends. I bought steel caps for each side and tightened one on permanently. I took a can of butane and found one drill bit the same size as the o.d. of the canister's tip and another the same size as the i.d. of its tip. I drilled through the center of each cap with the smaller bit. On one of the caps, I took the larger bit and drilled halfway through the first hole I made. That cap then had a hole that the butane canister tip would fit into and stop when it reached the smaller diameter hole.
First, dry the pot completely. Fill the tube with pot, tightly screw on the removable cap and stick it in the freezer until as cold as it'll get. The freezing step can be skipped but helps. Pull it out of the freezer, stick the tip of a butane canister in, aim the other end at a ceramic bowl and finally, press the can and tube together. The butane will fill the pipe, spray into the bowl, dry almost instantly, and voila. Perform in a well-ventilated area and wear gloves as the tube will get freezing cold even if you don't freeze it first. Allow some time for any remaining butane to evaporate before smoking. Warming it a little will help it cure faster.