First off I always use synthetic oil and only change it in the fall even if I put 100 hrs on that season* One time I did 140 hrs but I did change it halfway thru the season at 70 hrs.

What I have been told to do by my Supra Mechanic is this, if doing alot of surfing add half a liter more of engine oil to make up for excessive bow lift and sideways tilt and also use about a pint extra of transmission fluid in your transmission

I use synthetic in transmission fluid also ( Red Line)

Hope this helps>> Dave