The reason you are supposed to use 25W-40 Mercruiser oil in an I/O is that an I/O runs hard, its not like a car with a transmission where it shifts and the load is reduced through a gear reduction. The heavy oil keeps the motor properly lubed under stressful loads, such as when on plane. If you were to run 10W-30 in an I/O, you would experience a lot of lifter noise at higher RPM, at that point, the motor is at a higher stress rate and the oil just won't stand up to the pressure.
When I had my boat in the shop this summer, my mechanic asked me what type of oil I ran, I said 25W-40 Mercruiser oil, his reply was great, don't run anything less.
I guess it all comes down to spending a little more $$ now on maintenance, or a lot later.