snip
Hmmm - I'm inclined to think that the oil system (if in good fettle) is designed to offer certain minimums at slightly lower than idle speed. There's either a useful oil film thickness there or not - I doubt the middle ground is particularly wide. This is just intuitive reasoning, and someone more qualified is welcome to chime in...
snippetysnip
S'very interesting. And you've clearly investigated this in depth!
On my next tankful of petrol I'm going to use the gears more, and drive like my wife a bit (hang in third until 38mph) - and I'll see if *anything* happens to the consumption ;-)
Cheers all - interesting stuff!