Ok, for what this is worth I think i've got egg on my face (not for the first time...)

I've bought a new WR250R for my work. My dealer informed me in no uncertain terms that if Yamaha found that I'd been using any type of synthetic oil under 10,000km that there would be no warranty covering oil useage, should it happen.

So, a synthetic oil freak has been banned, and the bike went off to work in temperatures ranging from 6degC to 42degC using Yamalube 15W-50 mineral oil. Frankly, I couldn't tell the difference. Whether it's the liquid cooling being kinder to the oil than my old TTR250, I don't know. But I now believe you can use any rated oil and you bike won't care as long as it's dumped frequently.

Not fully converted, I must admit, but it's shaken my synthetic belief.