I use a little more after run than a few drops; basically I put the throttle on idle and fill the carb. Turning the prop sucks it all right in, and I turn the prop over maybe 6 or 7 times to let the insides of the engine get covered. Right now I have Hobbico brand, 'cause I added it on to a Tower order. You can use air tool oil or automatic transmission fluid for cheaper, but so far the $3.00 bottle has lasted many days at the field and isn't close to being empty yet.

And yes, synthetic oil doesn't varnish like Castor does. I knew about the varnishing, but hadn't actually seen it yet. The varnish will also form on the insides, too. It's protection for a while, but I guess at some point it needs to come off?