Thought most modern vehicles use computer diagnostics to analyse oil plus time period to work out service period
I know when mines has just been serviced, it will tell me I've got 35000 miles and 2 years until next service. 15000 miles and a year later it's telling me a service is due.
That's great if you have a modern vehicle, Mark.
But those of us who have old, pre-computer engined vehicles don't have that luxury...

An old flame who taught me how to drive was obsessive about checking the engine oil, water etc.
Oil in all his, and my, vehicles was changed at least once a year and he always bought top quality.
He was paid as a test driver for a year working in teams of pairs driving 24x7 for DeLorean.
They went all over the UK in the test DeLorean before the company went bust.
Guess it's all to do with how much mechanical knowledge you have and with what types of engine.
He always owned high end sports cars, so I guess his approach to engine oils and what/how/where to use them might be different to Trev's approach where you're servicing family cars and lower value vehicles?
But he used exactly the same 'servicing' approach to all my cheap and cheerful cars I owned at the time, so I suppose a lot is down to experience as well as personal preference?