The only test I have seen indicates not much difference at all between motorcycle specific oils and cars oils in terms of viscosity breakdown. basically a good oil (car or bike) is a good oil. Change it regulalrly first, then worry about car or bike oil next.
Any car oil that is an oil fuel conserving oil is not suitable for bikes as it may contain friction modifiers. Any oil of 50 weight or above does not have to conform to these standards, so is a safe bet, 40 weights or above are probbaly ok.
The normal good car oils are much better than the oils when the GSs were designed. The biggest difference I have found is putting on a decent big oil cooler...the oil has a much easier time and the mechanicals last better

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