Thanks for your replies.
HRHQ -- Since the manual gives an upper oil weight specification of 40, aren't you afraid that an oil with an upper weight of 50 could possibly get too thick at operating temperature? The Majesty engine really spins (high rpm) at crusing speed. Do you think there's a possiblity that an oil that thickens to a 50 weight might possibly impede the flow of oil through the engine?
Also, since the lower weight specification of 5W(for the Castrol) is lower than that specified by the manual, would this mean that the oil could possibly be too thin in the first few minutes of operation when cold to give adequate protection?
Of course the safe bet is to do what MyHrdly did and stick with Yamalube--at least to insure there's no warranty issues. I found Yamalube offered on a web site for sale by the case--costing about $4/qt--before shipping. It didn't say anything about being synthetic, so I presume it is just regular mineral oil. That's a pretty high price to pay for mineral oil. Especially when you can buy automotive synthetic oil for little more than that per quart.
Oil is an interesting, but complex, subject. I've heard Shell Rotella T is an excellent oil for motorcycles. Their regular Rotella is a 15W-40, so it would seem to fall in the correct weight range. However, it is a "plain" mineral oil. They also make a synthetic, however, its low weight specification is 5W, so once again, it might be a little thin at start up and in the first few minutes of operation. They have a web site with a lot of information on it and you can pose a question to their oil expert. I might do that and see what he says. If I do, I'll let you'all know what he said.
Thanks again---Bob