"Roberto, the impeller isn'
"Roberto, the impeller isn't very difficult to change, but a service/repair manual (such as Selocs) certainly makes the job easier.
In general, you should change the impeller every second season (whether you use the boat much or not). It is just made of rubber, and even just with the motor sitting out in the sun, the impeller can degrade or dry rot.
You don't want to "toast" a motor over a relatively cheap part. Also be sure to change the gear oil every year. Another cheap bit of insurance against destroying the gears in the lower end...
As far as oil, save your money and just use any brand of oil rated TCW-3 for 2 stroke (cycle) Marine application. The synthetic oil, in my opinion, is simply not worth the cost on a motor such as yours.
3 of my 4 motors are over 20 years old and I have used whatever is on sale (although I don't mix brands of oil in a given tank of gas).
This year I ran my "young" Mariner over 100 hours with Walmart's store brand and no ill effects (it is now 16 years old and is probably getting close to 1500 hours of use on it)...."
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)