I have a 97 single step 29 Fountain with a NA 540 that should be about 675 HP at the prop. I have a new Imco SCX upper with a -2 Imco sc lower. I also have a Drive Guardian. I plan on using a Hydromotive P5X prop. What kind of life should I expect out of this setup? How often should I change the oil in the gear case? The boat will be used in fresh water rivers and lakes. Thanks for the input.