for as often as your going to do it, bite the bullit, buy the yama lube and the yama filter while its still under warrenty.
keep a log book and all reciepts.
Is this series of engine ( F90, etc) as reliable and trouble free as advertised ??
Also, is it necessary to do exactly everything in the maint. chart( such as fogging in off-season - which I've never done on outboards) to maintain the warranty ? Is Yamaha real picky about being that specific ??