Dirty oil is no good for your engine so I go for cheap semi-synthetic (£12 per can) and regular changes.
I've been getting some hyper-mileages using this technique.
That's exactly what Americans do, it has to be especially good for cars doing long daily commutes on the freeway. Which is pretty much everyone in the US, or at least in California.
When I left France the oil change interval I was used to was every 10,000 kms and a (French
) car was expected to last 200,000 kms. The old oil I used to see from oil changes was very dark, almost black.
When I arrived in the US, I was surprised to see pretty much everyone changing their oil every 3,500 miles, i.e. twice as much as the French. The oil I used to see from oil changes there was a somewhat dark yellow, but certainly not brown yet. And, cars over there are expected to go 200,000 miles; if they're Japanese cars and they're well looked after in terms of regular maintenance, 300,000 miles is not out of the question.
Since I suppose Honda and Toyota use pretty much the same engines in Europe and in the US, I concluded that the more frequent oil changes (and more freeway miles) made a big difference.