I would say, given the cost of oil and filters against the cost of a motor, it is well worth changing engine and gear oils at 20 hrs. I have always done this as well as dropping the leg, greasing the studs (as well as removing the water pump housing to clean a lubricate the bolts) , , , , makes life much easier at the 100 hr/annual service and IMHO sensible. My Merc 150 4S definitely says 3 months or 20 hrs as the first 'service'