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Both my '65 337 and my '73 P337 seem to exhibit the same behavior (and this is my second set of engines on the '73).
Fill the sump to 7 quarts after an oil change. On the engines I've had (and have now), I see a fairly quick drop (sometimes just one flight) to 6 - 6.5. Another couple of flights it drops to somewhere between 5 and 6. On my current engines, the rear stops at just above 5, the front at just below 6. And there it will stay for 20 hours or so, with very slight drop perhaps, until the oil starts to get a little old, and begins to burn a little. I change oil every 25 to 35 hours (goes longer if I am away from home, I wait to get home to change it).
From this experience, and on the advice of my fright instructor/A&P who flew checks in Skymasters for 500+ hours, I wait for my rear engine to go below five before adding (I always add 1 quart at a time), and I wait for my front to get to 5.25 or so.
This is not what the POH says, so remember this advice is worth what you paid for it, and you make the decision whether to follow it or not. But it has worked for me for six years. Front engine on last set went to 1700 hours, was still going strong...
One exception. If I am flying in very hot weather, ambient on the ground above 85 to 90, I might add a quart at about 1/2 quart higher than the numbers above, just to keep the oil a little cooler during climb.
My two cents. We'll see what others have to say...
Oh, P.S. I still get small amount of oil on the belly and rear engine lower cowl. But nothing much...
Kevin
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