A humble question, thought there might be a sound technical reason for either choice I'm about to make.

Just hit 100 hrs on my 575 SCI, which normally means fresh engine oil and drive oil. I'll likley put another 3-5 hrs on the boat this yr before putting it away for 6 months.

Not talking about convenience, but what the motor and drive want to "see"

1- if I change fluids now then the fluids resting in the boat for the winter have 3-5 hrs run time- Is this hurting anything? Aciditiy in the oil is something I've heard before on this subject but not sure how real a deal that is.

2- Is it more beneficial just to run the existing fluids the rest of the season (24 hrs on it now), so that fresh stuff is in for storage?

Mobil 15-50 in the 575 and Merc hi perf in drive if it matters- and est 80-85% at cruise, 5% buzzing and 10% at idle.

If just 3 to 5 hours more, I would not change it until storage. Fresh oil for storage means come spring you are good to go. I do not like letting an engine sit for months with oil with combustion products (acids and carbons) in it just to change it out in the spring before start up. This is based upon storage in a dry condition (good shrink wrap if outside)

I would just wait to change them until you winterize. Another few hours won't make a difference. I always like to put my boat away for the winter with fresh engine oil. Drive oil, I don't worry about so much.