I would normally agree with you regarding the engine being run in but if a previous owner just took it to the shops once a week or crawled through traffic in urban areas the engine would not necessarily be run in correctly (many diesel engines with high mileage can be stripped down and still find the honing marks scored into the bores....suggesting that the engine has not been run in correctly). I have torn down engines with just 21k miles and found the bores to be completely worn but equally I have found engines with over 100k miles to still be as fresh as the day it was built. It all depends on how it has been driven. Another example is a BMW R1200GS motorbike I had for many years. When I bought it, it was consuming quite a lot of engine oil, I changed the grade but it didn't like it (just yet). The bike had only covered 24k miles but it was sluggish and didn't respond as it should. I rode the bike hard (as it should be ridden) with high revs in each gear and eventually it was a completely different bike. No longer did it consume oil but I could happily change the oil grade accordingly. The long and short of it is that it was not run in correctly and I managed to change that by riding it hard for a period of time. Most diesel engines are not revved hard enough to burn off carbon (which could be gumming up the rings into the piston and not sealing in the cylinder efficiently, blocked EGR, blocked DPF, etc. All they usually need is an 'Italian Tune-up' to keep it all clean. Moral of this lengthy reply is - if it is consuming oil between services then I would consider that my theory is correct. Also try a cylinder leak-down test which will show exactly which cylinder or valves have pressure leaking past them. Very easy test with visible and audible results