Your annual temperature range is -10C to 35C.
The oil viscosity of 15W-40 means that the oil is no thicker than straight SAE 15 oil at winter temperatures and no thinner than SAE 40 weight oil at 100C. Winter temperatures are defined as follows:
0W is -30C; 5W is -25C; 10W is -20C; 15W is -15C; 20W is -10C; 25W is -5C
Summer temperature is always defined at 100C for all viscosities.
This means that if 15W-40 for gasoline engines is not available, you can use 0W-40, 5W-40 or 10W-40. You will probably find that 10W-40 for gasoline engines is widely available and lower in price than the other grades and lower priced than diesel oil too.
If you check the manual for your car, it will most likely say that 15W-40 oil is recommended for all conditions and it will give a graphic chart that shows other grades of oil that may be used at specific temperature ranges. I think you will see that 10W-40 is approved for use in your temperature range of -10C - 35C.