I tend to agree with Vince on this. Over the years Royal Enfield has recommended 20w50, 50, 20, 15w40 and I am sure other oil viscosities for the primary. A lot of customers use ATF both type F which is supposed to be good for plates (i have notidea, but that is what they say) as well as Dextron etc. There are a ton of oils out there that use fear as a marketing tool - as in, "if you don't use our oil your mechanical device will go to hell in a handbasket.". For a while Lucas additive were the snake oil additives that the internet crowd swore there have been others. ( I am not knocking Lucas) All show impressive "research" that buttresses their claim. In my day you could buy very inexpensive "re-refined oil" which was what the name implies. I would not be afraid to use any of these oils because even the worst oil today is better than what was available back in the day. Having said that, I think the best thing to do is to use a motorcycle specific oil from a good company as suggested by Vince. If you want to start a bar fight try to tell someone that the brand they swear by is junk. I don't think it really matters much as long as you change it frequently. I do not want to disagree with a dealer, but I would be very surprised if the oil caused a primary problem. I have been wrong once before though. I think I have said it elsewhere on the forum, but for a long time the factory recommended 20w50. They changed it to 15w40 because a dealer in northern India thought it might be better for a cold start-up situation. There was ABSOLUTELY no science involved in the change of recommendation. I use 20w50 and that is what we tell others. As you have discovered 15w40 is hard to find in the US. As far as the primary is concerned anything south of heating oil is probably OK although we tell people to use 20w50, 10w40, 20wt, ATF etc. (don't use heating oil I am only kidding). In more modern engines the manufacturers recommendations become far more important than in these old designs. I will say DO NOT use straight 50wt. It was used in the old days because of fear of mulit-grade oils, to stop or slow down oil leaks and to help compensate for bad rings. It did none of these things very well however. Regardless of what your buddy tells you it is not a good choice.