G'day vint_mow,
I should mention here, that I have a significant technical background in lubricants; including wear metal analysis of used lube oils, for machine/engine condition monitoring programs.
Personally, I wouldn't go out of my way to insist on a monograde SAE40 here. Any decent modern oil is going to be way ahead of what was on the market back in the 1960's. I.e. as regards its tribological properties, detergency/dispersancy, anti-wear characteristics, oxidation resistance - the list goes on!
What we have here, is a small capacity, low specific power output [i.e. in kW/litre terms], air cooled, spark ignition, single cylinder side valve engine, with splash lubrication.
Penrite therefore make a very good point, when they recommend their SAE30 monograde small engine oil. A type which has worked very well in the very similar B&S and other US-made analogue engines, for many years now.
Yes, diesel service SAE40 monogrades were around from the late 1930's, and were formulated as OEM specified oils for GM/Detroit 71 series supercharged 2-stroke diesel engines. From the 1-71 single all the way to the 24V-71 V-24! 71 cubic inches/1162cc was/is the cylinder capacity.
As these 'Jimmies' always had fairly high oil consumption, any improved lube oil formulations have had to use low-ash additives, to minimise combustion chamber deposit formation. Lube oils for spark ignition air cooled piston aero engines have the same limitation incidentally, as oil consumption there is high too, by modern standards. Particularly for the radial types.
The thing is, there are more effective additives, that can be used where the 'low ash' constraint doesn't apply. Particularly on the anti-wear side. So a diesel rated SAE40 monograde is not necessarily superior to an SAE30 formulated for small engines, and may in fact result in shorter engine life, in this application.
