Just bought a '02 Mustang 6 cylinder.
It has 86000 miles.
Should i use the recommended 5-20, or would their be something better?

 

yeah 5w 20 will sever you well, just stick with that.

 

How do we know when to put in a heavier weight? when we have to put oil in the motor between oil changes?

 

iirc the oils will be the same weight when at operating temp. they are just a different weight/viscosity when cold.

 

Heavier isn't necessarily better.. for 'better' make sure you go for a higher quality oil & filter. As long as you have good oil & a good filter, the engine will be plenty well protected with how advanced oils are today. I saw a few other threads on oil & filters with some good advice on that stuff

 

IIRC, your engine is spec'd for a 5W-30 oil, but reguardless...Amsoil spec's the car to use either 5W-30 or 5W-20. I doubt you would notice the difference between the two weights unless your V6 made a little more valvetrain noise with the lighter oil.

Engine Oil
Grade 1......API*
Signature Series 0W-30 100% Synthetic Motor Oil (SSOQT)
SAE 5W-30 Synthetic Motor Oil (ASLQT)
100% Synthetic 0W-20 Motor Oil (ASMQT)
XL 5W-20 Synthetic Motor Oil (XLMQT)
All TEMPS......5W-30, 5W-20 [1]