Here is a good thread on Yamalube alternatives.
http://www.thehulltruth.com/boating-...troke-oil.html
One guy wrote; Take note that this is a 2010 write up and oil prices have skyrocketed the last couple years.
I pay $19.95/gallon for Yamalube from a local dealer here who has a drum of it in the corner of the showroom. Bring your own jug...fill it...go on your way. I probably could save a buck or two buying off brand oil but I've done it this way for years and never had a problem. No reason to take a chance.
IBoats is a good site for your Yamaha too;
Check this out;
http://forums.iboats.com/search.php?searchid=6753582