The Merc plus is also a synthetic blend while the premium is straight dino, both with additives. Pennzoil premium plus and xlf are both also syn blends. None of them say how much is syn though. I honestly think under 100 hp any good tcw3 oil will be fine. I switched this spring from Merc oils to XLF for price and availability and can tell no difference. I have read the synthetics and blends burn cleaner (less carbon).