I have a 21 SL 450 and changed the oil at 20 hours, then 70 hours. 20 for the break-in period. I split the difference on 75 and 25 hours. Had the oil analyzed and had 'significant' fuel dilution. We surf 75% of the time. Kids tube the rest. I thought I could get a longer interval, but apparently not. Will change at 25 hours and analyze again. See if it makes a difference.

21 Supra SL
19 Moomba Max