Contrary to popular belief, mixing multi viscosity oils does not give you a set viscosity, only a mixture of several different viscosities.
15W40
Not recommended. Use 10w30.
It could be.
Gas 10w30 Diesel 15w40
10w30 or 15w40
Yes, you can mix conventional oil with synthetic oil with no problem.
Gas 10w30 Diesel 15w40
mine takes 15w40 in the summer and 10w30 in the winter
Gas engine uses 10w30 Diesel uses 15w40
Gas engine 10w30 Diesel engine 15w40
5w30 or 10w30 for a gas engine, 15w40 for a diesel
15w40 is thicker at all temps vs 10w30.