[b]DSK:

Use synthetic if you wish, but a quick visit to your local drug store will get you a small bottle of USP castor oil, just dump two or four ounces in your gallon of Cool Power. The 15% is fine, I run the same for what that's worth.

No, the all synthetic will not hurt the engine, the ring will still seat to the cylinder by wearing to fit. There's the rub. Using castor oil the ring doesn't really wear to seat so much as it gets a varnish coating to match the cylinder wall. The cylinder wall will also get a slight varnish coating. Once these coatings are established wear is all but eliminated. Not so with synthetic oils, the parts just keep wearing.

Your choice.

Bill.