When my plane was new in 2002, after the initial run-in, it used to burn about 1qt in 15hrs.
Later on, for most of the years, it was doing 1qt in 8. This then worsened considerably after a tight succession of ~FL200 flights. Using the Mobil 1 0W20 method I got it back to 1qt in 5hrs, and an overhaul reported no unusual conditions inside.
My new engine started off 1qt in 1.5hrs (normal) and over about 50hrs improved to 1qt in 7hrs. Now, having done about 100hrs since April, it is using 1qt in 8-9hrs.
Relevant data points are:
So, I wonder if engines operated at say 75% power (or more) use almost no oil because the higher cylinder pressure ensures that the top ring acts as an oil control ring. I wonder if @ed2 is still around here…
Is your oil black or honey coloured after 50 hours?
You may be thinking of this but actually in the 2002-2005 years I was G-reg and paying £600 for each 50hr service, so I ran the oil to almost 50hrs every time 
When I went N-reg, 2005, I started running shorter intervals – usually… more like 30hrs.
Apart from running beyond say 40hrs, which I have not generally done for years, I don’t see the service interval length having a big impact, but “something” is causing the big difference in oil consumption.
Peter wrote:
So, I wonder if engines operated at say 75% power (or more) use almost no oil …
This is absolutely not my experience with engines of that size. When I was flying the C421 and C404 a lot (both have the same GTSIO 520 engines) they would consume the most oil during long climbs on hot days. A C404 at maximum take of mass out of Girona in summertime can require 30 minutes or so to climb to FL160, all the time running at maximum continuous power with the temperatures at the red lines and oil pressures in the yellow. The same in a C421 with 8 people on board out of Verona or Milano climbing at full blast to FL200 in order not to hit some granite. Each engine can easily lose 2 quarts of oil during such a climb.
On a cool day, climbing gently to FL120 and staying there for several hours (which would be the normal way to operate the unpressurised C404) will not consume any oil at all.
What_next, that’s very interesting; I’ve definitely seen the same effect (in smaller quantities thankfully) in the PA46, to the point that I don’t travel during the summer without planning my oil supplies as carefully as my avgas.
Since about half the hours my aircraft does is when I tow gliders, there’s a lot of short but high power operation flights in there. My engine uses about 1qt every 20 hours.
Worth noting the engine size. Oil consumption is per-cylinder and per-cylinger-size so a 540 should use 540/200 i.e. 2.7x more oil than a 200.
Well, 540 is 1.6 times larger (and 2 cylinders more) than an O-320-B3B. I don’t think there’s a direct 1:1 correlation between engine size and oil use, I don’t think a 540 will use 2.7x the oil of an O-200 for instance.
Back when I was in a club, the whole fleet used about 1qt every 12 hours +/- 1q/hr, and that was from a C150 to a Bonanza.
Probably the biggest oil use was when you got a new member who put too much in and it ended up getting blown overboard until the oil level was right again.
As has been discussed before, it varies a lot with the level you start at. If you too an engine up too high it just throw it out!
I tend to operate at 75% constantly and run 8qts in the sump of an IO540. Oil usage over past 5 hours has been approximately zero*. Dipstick in same place every check.
* obviously not really zero but unnoticeable
