I'm sure it's good oil old PJ, and probably better than most monogrades. Had multigrades existed when our bikes were built, quid to a pinch of **** it would have been recommended. Yes, it's runnier when cold, and there is the risk of slightly more leakage. However, for 30 years I've been using any branded 20/50 in all bikes with shell big ends. If I had a single with a roller big end I might go the mono route. I also use 20/50 in a pre-war car with white metal mains and ends and have for nearly 20 years. You WANT the oil to be thin when cold on start up so it flows . . . The only time I wouldn't would be if a motor had been run on mono for ages and hadn't been stripped and cleaned, for fear that the detergent elements in the multigrade might dislodge muck in the oilways. I think, personally, we are too often blinded by myth and less susceptible than we should be to reason. A decent multigrade can easily cope with roller and ball bearings and bushings too - heck, it was recommended lube for many a gearbox for years - and so I'm sticking with it. Far cheaper too when it comes to the regular changes we tend to go for - 1000-1500 miles is pretty much par for the course.
Finally, in some more recent cars (1960s) in my tatty pile of stuff that runs, I run them in on 20/50 and then switch to semi-synthetic Castrol 10/60 from Castrol Classic Oils - pricey - but I only change it thereafter at 5000 miles, as the stuff doesn't degrade like non-synthetic oils owing to the long-chain polymers and all that - so it ends up cheaper. A chap I know who used to be head chemist at one of the big oil companies told me that an oil like the 10/60 would be in a better state after 5000 miles than any non-synthetic after 1000. I believe him, and so far so good! Groily
