It's all about the oil. Sufficient volume of the right viscosity delivered at the right pressure at the right temperature with the right level of filtration.
A car engine doing 1650 rpm is doing 99k revs per hour, say 100k for easy sums. That gets you 70 miles. A reasonable life span for a car engine is 180k miles, so a total of 257 million rotations of the crankshaft in that life time. Of course, a lot of the time you're going to be doing higher revs in lower gears so let's round this up to 300million rotations, which represents in the order of 2,500 hours of driving. Of course, average speed for a car is more like 35 mph so it's more likely to take 5,000 hours to do the 180k miles.
Outboard motors can't be measured in miles but in time. To get 300 million revolutions out of an engine that spends its time at 4500 rpm on average is less than 1200 hours of service, which if Google is to be trusted is well within the expected life span of an outboard motor, which is around 4,000 hours.
So somewhat surprisingly, I'd expect your outboard to last comparatively longer than the car!
