Step two: so what was in there before?

Now you need to assess whether the previous choice of oil was a good one, that is, how suitable and how good the engine oil in the engine actually is. If you've just bought the car, you need to ask the previous owner, and while you're at it write down absolutely all the information about the oil (the exact brand and manufacturer). If the car's previous owner doesn't remember or won't give you that information, well, it's not the end of the world, but it does mean extra costs for you (for why, read the article on engine flushing).

If the last oil change was done while you owned the car, you need to track down absolutely all the information on the label. In any case, if you have no information about the oil actually in the engine, go straight to step four.

Another important parameter for our future search is oil consumption (burn-off). Given the very nature of engine oil's presence in the engine, oil consumption can't be zero by definition. Some amount of oil burns off no matter what — the only question is how much. For every engine, the manufacturer sets an average oil consumption norm, but that doesn't mean your oil consumption can't be higher or lower than that norm. Read more about this in the article on oil consumption in the engine.

At this stage we just need to work out whether current oil consumption exceeds the manufacturer's norm, and if it does, by how much. The best place to find out the oil consumption norm is an official service centre for your car's brand (if that information isn't in the manual).