We have the Nanni 4.22 and me and my dad always change the oil at the end of the season and change the oil filters but i leave the fuel filters inline and pre incase on the off chance i get diesel bug thou i do treat the fuel tank with the doctor diesel stuff and never had an issue so far.
My engine takes 6ltrs of oil, 2 fuel filters, 1 oil filter and i always check the air filter is ok and clean, the impela is in good condition and check the packing box is still ok. The fan belt was changed after 300 hours, i need a new battery for the electronics as after 3 years it just wont hold charge.
If your like me and find it hard to get the oil out with the drain plug being so low down, for £20 you can get this fantastic oil pump that goes down your dip stick tube, you pump it up till the pressure is there then let it suck out. Best to run the boat engie to warm the oil and i always put about 1ltr in let it settle then pump that back out, nott he best way as you wills till have old oil sitting at the very bottom of the sump but very easy.
Also turn the fuel and the water intake off during the winter, some people drain the water intake but i have never and it has been ok even when it was very very cold last winter.
Some of the other lads with more years of boat experiance may have better ideas but the above is what we have done and read about and so far no issues and she fires up no bother, thou i still hate doing the fuel line air bleeding.