Tom - I'll provide this feedback from another type club where the 390 has been commonly in use for more than a decade, hence this type club likely is the best clearing house for info on the 390 as deployed in amateur-built aircraft. The statement below is a single post which acts as a reasonable summary of a complete discussion thread on the topic of the 390 and oil consumption. It is generally held that the 390, on average, consumes more oil than the 360's in this airframe.
Hope this helps.
I have the IO-390 in my plane -- and love it! The engine has given me no trouble since I built the plane in ... October of 2006. As of this moment, I have 1297-plus hours on it. The engine has had great compression checks at each annual condition inspection. I
buy a box of oil (12 quarts) every 50 hours -- which reflects my total oil consumption.
When I change oil, I put in 8 quarts. Thus, over the next 50 hours, I will consume three to four quarts of oil. That's a consumption rate between 12 and 16 flight hours per quart of oil. ...
I've never had to use more than 12 quarts in 50 hours. Generally, I use 10 or 11. In recent years, I just set the extra quarts aside and figure that when I've got 12, I won't have to buy a box for the next annual.
I know from experience that the engine will consume fairly fast if I fill above 7 quarts so other than when I change the oil, I fill to 6 1/2 (i.e. when the oil drops to 5 3/4 or so). If I fill to 7, the engine consumes down to 6 1/2 quarts in the first flight. There after it stabilizes the consumption rate. If I thought that it was necessary to keep the oil level at 8 quarts then I might get the kinds of oil consumption rates that some people... are suggesting.