My final comments on oil changes:
Putting 5l of oil through the refinery, packaging transport and re-cycle process is greener than 10l in two lots.
There is a lot more to modern oil specifications than the viscosity rating.
The colour of the oil is not an indication of it's condition. For example oil in a diesel engine goes black in few hundred miles but the alst diesel engined car I owned had a 35,000km (21,000 mile) / 2 year oil change interval. That engine will easilly do over 200,000 miles.
I totally agree that today's synthetic oils, regardless of viscosity, are a far cry and vastly superior to the mineral oils that my parents and I grew up with. In those days with 6V charging systems and winter temperatures you used a straight 20W or even 10W if you had any hopes of the engine cranking. But as soon as the warm weather hit out it came and straight 30W or 10W-30 went in. And back then oil change intervals were as little as 2K miles. The Germans found out this little fact the hard way in the first winter they had invaded Russia.
Also agree, especially in diesel engines, that oil color generally does not indicate it's condition. But in a gas engine totally black oil I think is a good indicator that it should be changed. But in reality the only thing that can tell you for sure is an oil analysis. But who's gonna pay for that every time you think it needs to be changed?
Actually, rail road operators DO rely on oil analysis for their diesel engine fleet. The engine oil is never changed unless the analysis indicates an issue. But if it does indicate a problem chances are the engine must be torn down and rebuilt. But under normal circumstances they only add oil to make up for what is consumed.