Pontoon Boat & Deck Boat Forum

I have a hard time finding 25W40 FC-W oil for my Mercury 4S 90, especially the Quicksilver brand. Ran across Bel-ray which is FC-W certified. But I don't see many reviews of using it.

What puzzles me is that all the Quicksilver oil labeled for "outboard" is 10W-30 and "inboard" is 25W-40. Yet Mercury's website says "Mercury recommends 25W40 in all cases except the small outboards at low temperatures".

I'm still in warranty so I think I should stick with 25W-40 FC-W oil.

When I had a 90 4stk my dealer said in Okla or warm climates 10W30 was fine.

My dealer quietly told me just plain high quality automotive 10W30 would do just fine. Our temperatures are fairly stable during the boating season.

After reading in the marine section on BobIsTheOilGuy.com, I chose to use Chevron Delo diesel oil (universal oil). As long as it's not going into anything with a catalytic converter, it's fine in a gas engine. Many of the ultra high hour fishing fleets use diesel oil in their big 4 stroke dual motor usage in Florida because it's cheap and does just fine.

Now that I'm up to 40 hours, I'm probably going to changeover to Mobil 1 EP--which is substantially better oil than plain Mobil 1.

We got our Bennington in Feb. '08. I was new to boating. I spent awhile doing some research on oil and I have to apologize for not being able to locate the page for the test. It appeared that the 10W40 Pennzoil was the winner for common Dino oil at containing the best rust inhibitors. I've been using it every since with ~400 hours on the F225 Yammi. I use Shell Rotella T 15W40 in the Diesel 6.4 liter and in our 26HP Kohler Courage Pro motor on the Husqvarna Zero Turn mower. Some insist on full synthetics. I think I would use Amsoil if I chose a synthetic. Good reading can be found at http://www.carbibles.com/engineoil_bible.html and http://www.bobistheoilguy.com/motor-oil-101/

I use 15W40 diesel engine oil.