Lacquer thinner. Cheapest stuff I can buy. Use a lot of it and flush out ALL the crude. Amazing how much is in there. I've seen it run near black the first couple of slugs of thinner. For real fun, Try flushing an old Chevy suburban with rear air. Must be a mile of hose. (actually there are two hose over 20 feet long!). Doesn't take much to foul up A/C so make sure it is really clean then pull a vacumm on it overnight.