Interesting debate. I run a 400 in the UK. Looks like I ought to use 15-40 Diesel oil during the Winter and a 20-50 in the Summer.
I surely would! But then, the Baby Fours will feel the extra drag of the 20w[n] base oil more than the larger engines. For a long time there was a 15w50 oil available in a non-detergent diesel formula (here in the USA) from Shell (Rotella), which would likely be better for the small ones in hot weather. Viscosity numbers are all about the temperatures: when I was last in UK it was both hot and humid, and I remember thinking about my 750 then, and which oil I would have used (but I didn't have the bike with me, dang...). Back in the 1970s, Castrol made a 10w50 (which I used for my 750 in the wintertime) which I used to sell to the 3 CB350F owners out of my shop, and those bikes ran like (mechanical) Swiss watches!