I have a 2004 Yamaha F90 Four Stroke outboard and have used Pennzoil Platinum Full Synthetic Oil in her since her 10-hour break in.
My owner's manual (page 1-13) did not state anything about using a "marine formulated" oil. In fact it just states an API "SE,SF,SG,SH,or SJ" rated oil 10W-30 or 10W-40 depending on the temperatures you operate the boat in. The Pennzoil is rated API "SM" and I have 478 trouble-free hours on her - My boat is the one that stays at the "camp" on the bayou so EVERYONE uses my boat...and my gas! One other CRAZY thing is that the owner's manual states that the oil filter cartridge is to be changed every 200-hours by the DEALER. Are they NUTS??
Spend the $14 every 100-hours AND DO IT YOURSELF!
Having done the oil change 5 times already, let me give you a pointer. If you try to remove the oil filter from the engine, even after letting it sit for 30 minutes, it is a big mess, you will spend 30 minutes cleaning up spilled oil! Do this - Drain your oil, then take a nail or a punch and puncture the top of the filter. Take a Mardi-Gras cup (For those of you NOT from Louisiana - just use a disposable plastic container!) hold it under the filter while you turn it 180 degrees and drain the oil from the hole you punched into the cup. You will still have a little cleaning to do but nothing like the mess I had during my 10-hour initial change!
Watch for the sales and rebates on the Pennzoil Platinum Synthetic Oils, I think I've cashed in on two or three of these rebate deals.