Yes, you can mix conventional oil with synthetic oil with no problem.
if it si 15w40 engine oil
Not recommended. Use 10w30.
Use 15w40 motor oil unless specified otherwise
10w-40
Castrol 10 30
Can be either Mobil 1 0W40 or Any name brand 15w40
I use 15w40
10W40 or 15W40
15w40
A diesel engine would use 15w40 a gas engine could use 15w40 or 10w40
I use 15w40