any of those above i would be happy with personally, i wouldnt ever go down to a 0/30 or 5/40 like some people do though, its my belief that this is too thin for the bearings on a YB

having never really used a YB in any real anger on trackdays or anything though whilst comparing different oils, my advice is only my opinion and not the result of experimentation, so i would suggest asking Harvey, especially as he is the one who has built it.

or PM oilman and ask him to contribute to this topic??