15-50 should be acceptable.
Oil for diesel engines is also a good choice.
Stay away from energy conserving oils, they have friction modifiers that make the clutch slip.

15 wt. fork oil, ATF in a pinch.

Welcome Andre,

You living in England I'm not sure you would need 15-50, unless it gets really hot there. These are just my opinions; when the outside air temperature is cooler I use Castrol 10W-40, and in the hot summer I use Shell Rotella T 15W-40. No matter what folks will say, I have always avoided synthetic oil in the crankcase.  In the past, for the forks, I have used ATF, Bel-Ray full synthetic 10 wt fork oil, and Castrol 10wt fork oil. All have worked well. In the summer of 1988 I spent a wonderful three months at R.A.F. Sculthorpe near Fakenham and parts beyond and have fond memories of that area. Regards, Jim.

Best to use a good synthetic oil made for motorcycles,You pay a little more but in the long run its cheaper no motor repairs,I use Klotz racing oil 15w 50w and Klotz fork oil 15 weight.

The reason I use a decent quality conventional motor oil is because I think, IMHO, is that synthetic motor oil is a complete waste of money. I own 3 motorcycles, and one has 48k miles, one has 24k miles, and the humble Savage has a smidge over 8k miles. Again, IMHO, unless your "wringing the tits" out of the machine, there is just no need for it. There, I said it, let the bashing begin. Regards, Jim.

I am in Oklahoma. Very hot and humid summers. My brother in law and I run more than 4,500 miles every year. Lots of superslab miles at speeds between 75-90 miles an hour. Often times my wife is on the back seat, and the terrain here is anything than flat. That motor gets stressed hard like that. Hardly ever the tachometer is below 5,000 rpm. Up the hills is full throttle often, especially with two up. Like I said, 105 degrees and 70%+ humidity for several weeks in a row is not uncommon. I think that qualifies. A close to 4" stroke makes for some pretty high piston speeds, no doubt.
I used to run Lucas 20W-50 in summer, Mobil1 10W-40 in winter and recently switched to Rotella 5W-40. This will be the first summer with this Oil. I intend to run that oil all year round. With the Lucas, oil consumption wasn't worth to mention. A couple of shot glasses maybe every 1,000 miles. Because less than 20 bux for an oil change, I threw it out every 1,500 miles. I surely doesn't hurt anything. We'll see how it works out with the Rotella. I wanted to give it a try, since countless people brag about it. The bike has almost 15,000 miles down, and all it has been done to it as far as "repairs" is valve gap check three times, where the first one didn't require much adjustments, they were still within specs, but on the upper end. The two following checks were actually a waste of time. Nothing to adjust. The other thing was the cam chain adjuster plunger was replaced at 11,000 miles with an extended one. Fairly early, I know, I have no idea why. Maybe 10 minutes ride to work every day, winter and summer did it. The bike doesn't even get warm like that.

oil is like is likie butt wipe.many avantages all are good,  i personally  iife ac delco 10-40  gpt 98,000 miles on my last thum[per  gn400
i also use it inall my machines.

oil is like oil everybody's got thier prefference, who am i to begrudge so one els' one's choick.

there are sevral bobber hatters and they express thier express those thougths

personally i would neve bad mouth somegbody-s choice.  so how bout keeping the "pos' thoughts to your self's  or better yet  post a pic  so we could critize your work o art.

I don't understand all the hype on oil for a bike. Once again, I have used only auto motor oil in my bike. Usually Castrol 10-50. I have 47,000 miles on it and it runs just as good as new. Most of it freeway so high RPM's. Change it every 1,500 miles or so, or after a long hard ride. I'm beginning to think all oil is basically the same. I'm beginning to think everyone wants us to spend a bunch on oil unnecessarily Gas today was $3.31 a gallon at Texaco, and $3.01 at Arco. My car runs no better Texaco gas.