Do you think the S.A.E. 30 may in fact be the best "compromise"?
Definitely yes, as a convenient, workable solution.
After a wee bit of thought [meh, it's an incurable habit with me], it has occurred to me that the SAE40 Diesel spec oil is probably what Southern Cross specified for use in their small diesel engines at that time, which was one of their major product lines.
And as their target mower customers were very much their existing customer base, they would have seen no reason at all to want their customers to source/stock a mower-specific engine oil, when one that these customers already had on hand, would work just fine.
