Far be it from me or anyone else to tell you what oil you should use, or how much you should spend on it, or what brand/grade to use – it is entirely up to you. However, broad statements like

are not really conclusive proof and a little misleading – it doesn’t necessarily mean that car oil is fine in a bike, or should be advised or recommended, just that it worked for him. The point being all good motorcycle oils are thoroughly tested to JASO MA2 standards on wet clutch engines/gearboxes and, as such, are effectively guaranteed not to cause problems; it’s the safe option – you know what you’re getting – they all work. The same cannot be said for car oil. Sure some car oils may work fine, but not all car oils are the same and some won’t. Some brands/types might work in certain applications (Hayabusas) but not in others (XTs). It's all a bit hit-and-miss.

It is well known and documented that “energy conserving” car oils are a disaster for wet clutches. But the thing is, even with ordinary car oil, most manufacturers are very vague about the composition of their oils and, even if you look up the product data sheets, they don’t really ever declare what’s in them. Castrol Magnatec is a good example of such a car oil. There is a lot of pseudo-scientific mumbo-jumbo talked about “intelligent molecules” and “clinging to your engine forming an extra layer of protection during warm-up and beyond”, but nowhere do they (Castrol) tell you what these molecules actually are, or what they actually do! It’s all a bit of an unknown quantity – a guess, a gamble. They sound a bit like the dreaded friction modifiers in EC oils, but who knows or dares to say? It would worry me that these magic molecules would be clinging to my steel clutch plates and providing an extra layer of slippery protection where it’s not wanted!

Friction modifiers aside, most car oils cannot hold up to the constant high pressures of meshing gears in the transmission either and the oils often prematurely break down and their service life is greatly reduced. Unsurprising really as they were never designed or tested with this function in mind. Another aspect of oil performance in which JASO spec oils are tested to a specified standard.

If you’re trying to save a few bob because you think you don't need bike specific oil then fine, it’s entirely up to you, go for it. You might find one that works well; you may get lucky and enjoy many miles without trouble, but on the other hand you might be unlucky and find one that doesn’t work so well. By choosing a JASO spec oil you are simply eliminating the element of chance/luck/guesswork and maximising the opportunity of enjoying long and happy engine and clutch life; it buys you confidence and that’s worth a lot, and all for just a few quid extra every few thousand miles.

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