Castrol has a 5W50 synthetic that is for older cars. I think this would be fine for bikes that didn't have wet clutches.
To throw another curve into this discussion, not necessarily so.
Is the oil in question tested to be stable at the higher temps found in air-cooled engines? Also, is the "detergent package" (a non-scientific term I have just invented to cover the additive chemicals put in oil for specific purposes - the relevance of which will be clearer in the next paragraph) appropriate to an air-cooled flat twin?
Some years ago I bought a grey-market import Toyota Surf (turbo diesel Hilux to English and "Tundra" (I think) to US readers. The engine package (2L-T) was semi unknown in Australia - the L series diesel engine (sub-port injection, dry liners) was well known as it was fitted to Toyota and Hino vehicles since 1968, but the 2L-T threw in the curve balls of 1st generation electronic injection control (a crock of Sh!t guaranteed to use more fuel than an aneroid controlled injection system) and a turbocharger that gave a slug of an old diesel in a light truck the "get up and go" to stay with passenger sedans in traffic and the ability to cruise at highly illegal speeds on the highway.
Anyway the Australian arm of Toyota decided that it would offer no assistance to owners of grey-market import Toyotas so user groups (in some ways similar to this one) sprang up to share information and expertise. As you can imagine one of the first matters that cropped up was "what oil do I use"?
Because of the way I am wired up I contacted Shell, Castrol and Valvolene and asked them, they contacted their overseas branches and I was quickly told by Shell to use 15W40 Rotella, by castrol to use 15W/40 "Delo" diesel oil and by Valvolene to use either the mineral or synthetic versions of their 15W40 diesel oils (interestingly Valvolene told me to use Castrol Dexron III for the auto box and transfer case in opposition to their own supposedly Dexron III equivalent trans fluid, but would not tell me why.
Anyway I ended up in a huge argument with a chap who was convinced that Mobil 1 was the perfect oil for the 2L-T engine because his brother's wife's 2nd cousin, who once swept the floor at the Mobil refinery said that he once overheard a conversation that might have indicated that this was the right oil for turbo diesel engines.
His principal argument on the topic of the superior performance of Mobil 1 in 2L-T engines was that when he changed his oil, the old oil came out just as clean as when it went in.
I responded that the 2L engine was well known in the trade as a "dirty diesel" and that it needed high detergent oil with a sophisticated additive package so that it could clean carbon and combustion byproducts out of the internals of the engine and then carry them until the next oil change. I ventured to suggest that the fact that the oil dropped from my engine was as black as the dark side of the moon was an indication that the oil was doing its job and that if his was coming out clean then it was not capable of doing the job.
As can only happen in internet forums facts and logic became the first casualties of a one-sided war in which I was the enemy who knew nothing about oils. (Not that I had ever claimed to know anything, I was simply smart enough to write to oil manufactures and ask them what they thought)
Interestingly I am still driving my Surf as the thing simply refuses to die and I can't work up the interest necessary to replace it. My "opponent" on the other hand was the instigator of a lengthy thread a couple of years later on the topic of obtaining a referral to a good but cheap engine rebuilder as his Surf's engine had blown up. As this had happened to a number of other forum members and that the cause had been isolated to the use of incorrect oil, someone suggested that his beloved Mobil 1 might have been responsible. That suggestion was not well received....
The point of this is to gently suggest that there is more to be considered than just the weight of the oil to be used and that either the manufacturer of the engine, or oil companies should be asked to recommend (in writing) the oil they think should be used.
