There should be no problem with mixing them.
Yes of course you can mix them what what would be the reason to do so.
A diesel engine would use 15w40 a gas engine could use 15w40 or 10w40
10W40 or 15W40 semi-synthethic
10W40 or 15W40
10W40 is a thinner oil, and would allow the engine to run more freely, and although both are suitable for winter usage, a thinner oil will usually help the engine start more easily in cold weather.
15 is thicker oil and for bigger engines.
not recommended
10w40-15w40
15 is thicker oil and for bigger engines.
Yes, you can mix conventional oil with synthetic oil with no problem.
No, use only the weight recommended by the manufacture which is 15w40. I suggest you use a Synthetic oil on this air cooled 4 cycle engine.
probably not but 10W40 is recommended