only way to do it is look in ur owners manual and look it up. we sell oil and we get the same damn questions everyday and that's what i tell them 1st.
2nd is climate condition. over here it never gets that cold so we don't need a lower viscosity to cope with the cold start.
3rd condition of driving and load condition. u race the car? modified? carry alot of weight? mountain driving? alot of city driving? things like that.
4th age/milage of motor plus if it has been overhauled b4. (people here suck at rebuilding engines and should be shot).
my manual suggests a 0w-30 or 5w-30 for synthetics and 10w-40 to 15w-50 for cheap petroleum based oils.
i use 100% (PAO) synthetic amsoil 10w-30 or 10w-40 (depending on stock). changed at 10,000km or 6,213 miles.
like i said weather, it doesn't get that cold here (10c or 50f is the coldest).
then i ripped out my rear seats and it is full of stereo equipment, so it's heavy as phuk. then it's city driving sitting in traffic.
this is the 2004 mazda premacy with the 2.0 fs-de engine