Yamalube actually offers conventional (mineral), semi-synthetic, and full synthetic oil. However, 10W40 is only available in conventional form. As such, it does not meet the manual's requirements. I'd take the ski back and make them change it an oil that meets spec and ask to see them pour the new oil in (to prove they changed it). If not, I'd be very willing to be they will just say they changed it and hand it back without touching it.