Home Forums Fishing Freshwater Oil, and Filter for Four Stroke?

Our Mercury 20 horse seems to call for 25W40 oil. Should I go ahead and buy from a Merc dealer?

The short answer is yes. Beyond that, marine oils don't have to worry about meeting specs that automotive engine oils need to meet, specifically lower anti-wear properties needed to protect catalytic converters. So marine oil can be more made more robust than automotive oils. Also, because of the obvious water environment, marine oils are typically formulated with more rust inhibitors.

Here's a short write up on the new Mercury oils - Click Here

Walmart carries the oil here, it’s $10 a quart.

Or you could run 15w40 diesel oil in it. Tons of people use rotella or delo in 4 stroke outboards and motorcycles. Costs WAY less, works just as good. Cold weather starting will be better with a 15w oil. Diesel oil has all the additives that they took out of car oils like zink and phosphorus and that's good for engines that are under a load like outboard or engines that share a sump with the transmission like motorcycles.