I use Pennzoil Premium Plus, sells for about $5 sitting alongside Merc oil at $10 both per quart on the sporting goods shelf at wally world, and both meeting or exceeding the requirements of TC-W3 (Two Cycle - Watercraft, 3rd and current addition). PPP is semi synthetic....dino for rust protection and synthetic for clean running and "low smoke". I used it in a 90 and 115 Merc variable oil systems, starting off at about 80:1 at idle increasing oil ratio to 50:1 at WOT or somewhere along the way so says the service manual. Seldom do I see any smoke and find myself pulling the dipstick out to check to see if I am consuming oil.
The big guns on here living, using and associating with folks and their equipment in Florida mentioned the necessity of the dino to prevent rust problems so there you have it.