On my end, i've undergone a little bit of training at an oil facility in Melbourne, Victoria, Australia.
For starters: to be US friendly: - 1 kilometre = 0.621371 miles. 120,000 kms = 74,564 mi | 60,000 kms = 37,282 mi
- Left = Wrong side. kidding
Weights
1) Always refer to the vehicle manual to determine the recommended oil when it's new. e.g. 0w20 or alternatives 5w20 or 0w30.
2) Discover kilometers (sorry Americans; which is everyone ) since a brand new engine e.g. Travelled 110,000kms
2a) Every 60,000kilometres, go UP an oil grade e.g. 0w20 to 0w30 to 0w40 and onwards.
2b) In this example, 110,000kms = 0w30. When it hits 120,000km, recommend 0w40 as a starter.
3) Disclaimer: Some cars are in even better condition and the 60,000km marker is a guideline. It could be more.
Specifications
1) Always refer to the vehicle manual to determine the recommended/required specification.
1a) Regardless of age, it always stays the same. e.g. An Audi A4 requires VW 502 and VW 505
2) Check Oil for the matching specifications. If it's not shown, i'ld be a little wary about using it in the vehicle.
2a) Most people would say it's alright. Its not my car so if anything goes wrong, the business supplying the oil can be liable.
3) In the case of API, any rating after will be A-ok if nothing else more specific is specified. like a SN used when the car specifies SJ.
Base Oils
We don't need to go to that level however it's useful to know the various types.
In my case, I service 2 brands, Volkswagen & Audi and use 2 brands of oils. Certain car models should only use certain oils within the 2 brands. My life is easy and customers are guaranteed oil that is top notch, good quality, compatible and recommended by the manufacturer. Doesn't get easier than that! A little bit of research is required at the start but it's work worth its weight in gold.