What oil and filters are you running on this engine? The engine says to run SAE 40 but I was thinking maybe a 20w50 for the hot days at havasu. Are you using the stock oil filter or a fram oil filter.

I had the 500EFIs in 2 different boats and I used the Mobil 1 15w50 synthetic oil in them without any problems. As for the filters, I used Fram oil filters and Napa gold fuel filters. I have no idea what the number was on the Napa filters, but I believe the number on the Fram filters was HP5.

I ran the Mobil1 15w50 for over 400 hours and the motor still looked great on the inside. I use an AC Delco PF932-C oil filter. Its twice the suze of a standard filter, has very good filtering and flow characteristics according to hydrocruiser's research and holds one more quart of oil in the system than stock.

Mobil ONE

mobil 1 15-50

mobil 1 filter M1-302

mobil 1

In the last boat that I had, I ran the Royal Purple 20w50 and really liked that.

Amsoil Series 2000 20/50

i use Royal purple 20w50 and a stock merc oil filter

dean gellner might chime in

some wieght penzoil and a fram hp? filter was all he would LET me run