It would be useful to know what this is on. As a generalization, I would not want to run small engines w/ 15 w anything below 20 degrees. Are you hopped up on zinc is why you are looking at diesel oil? SME manufactures oil blend will give you that if you are worried in an old IC. If this is on winter equipment in the great state of WI, and you wanted to use a 40 weight oil, might I suggest a synthetic 0 or 5W -40? That old IC doesn't want to run up dry nor will it bake itself w/ a 30w oil in the winter. I cant say that I recall a base idle designation given as to an aluminum vc iron before. How do you arrive at that?

It is on a Simplicity garden tractor. Yes the zinc and the fact that diesel oil handles heat better. I do not like synthetic oil. And the idle speed designations are from the Briggs and Stratton website and my service manual (1981)

Does this engine seem to work better w/ 40 weight? If you are OK w/ 10W30- the additive package in the Kohler oil is regarded well. Base idle considerations are typically configured for what the engine was spec'd for. At least that's the way I look at it. It would be interesting to know if there is a reasoning between the two if all other things were equal.

Cast iron holds better compression and expands less than an Aluminum block when cold.

As far as the oil. I have been running Castrol GTX 10W30 in every gasoline engine I have for the past 20 years.

SAE 30, all of the time! Everything runs great-all the time! Nothing has gone "BOOM" at any time! JMHO