For What it's worth, . . .
I found the following here;
Yamalube 2M??? - iboats Boating Forums
Yamaha 2M oil contains a healthy amount of PIB (polyisobutylene) with is the low smoking synthetic
additive that is also in Johnson/Evinrude XD-50, Pennzoil semi-synthetic, and Merc's Premium Plus.
Each oil has varying amounts but all make the oil a semi-synthetic.
PIB replaces the bright stock found in regular TCW 3 oils. It is being used alot in both air cooled and marine oils in the last few years
to bring them up to the latest specs while controlling smoke and deposits. I have found some PIB
blend oils to be actually cleaner than straight ester oils.
I'm not sure he means that this is Marine grade I think he is saying that PBI is in both oil grades.