Originally Posted By: AjsGarage
Hope you havent changed the oil yet. If Yamaha says you need a FC-W 10w30 synthetic then thats what you gotta put in to stay in warranty. I have heard of a lot of people being denied warranty due to not using the right oil. Outboard manufactures are not as lenient as the automotive guys.
Here is their reasoning behind being so specific on oil:
Why you need the NMMA FC-W approval. Mobil 1 doesnt have FC-W approval
-Automotive oils have "less" corrosion protection then FC-W oils are supposed to have. NMMA has certified these oils with the FC-W approval for marine use. Just like you trust API ratings for auto use, NMMA is your approval body for Marine lube
10w30. Same as with a car, under warranty, you follow what is on the oil cap or in the manual, because they say its the best for that engine. After warranty, you can put in 20w50 or Ow20 for all they care.
Synthetic - Your engine has a catalyst. Marine synthetic lubricants are designed to work well with high friction engines such as flat tappet, and work well with catalysts. You put a high zinc diesel oil in, and you could possibly plug the catalyst, for the same reason zinc was reduced with API SM, SN. You need an oil that is catalyst safe, but willing to withstand 6000 RPM of continuous use.
For the most part, you can use any HDEO for marine use, EXCEPT when under warranty. For your warranty, stick EXACTLY to the manufactures specs. Or you may be out another $8000 motor if anything goes wrong and they deny warranty
This guy is on the right track, there are reasons FC-W approved oils are suggested for marine outboard applications. Using a diesel oil will almost certainly lead to phosphorus contamination of your catalyst.
The other main differences between regular PCMOs/Diesel oils and oils with FC-W approvals are FC-W oils are designed to resist corrosion better and also operate at higher temperatures.