'The Old Lawnmower Club' website in the UK has an owners manual for the Mk6 Ransomes Minor for download HERE
Except for the roller drive details, most of it should apply to the Mk7.

This states that the fuel/oil ratio is 16:1 for these engines, using 1950's oils.

The problem with changing this ratio, is that doing so also affects the fuel/air mix ratio going into the engine. Reducing the oil proportion will tend to make the engine run rich.

So it's not advisable to change the fuel/oil ratio too much; I'd be inclined to try 20:1.

Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."