I am ordering spare impelors for a 4hp and 9.9 yam, and a 55 suz just to have on hand for end of season change. Have never changed one before and I see they come with or without a set of gaskets. Much more $$ with the gaskets. Is a gasket change every season the way to go or is just changing the impellor ok. Also, Iboats sells a generic brand, is that ok or stick with the name brand replacement parts??