We have been over this issue in the past, and I have reviewed the archives, but with a lot of new Legacies flying, it is time to ask the question: what is the optimum oil level for the IO-550?
The reason I ask is that I had an incident during final when my MVP 50 engine monitor lit up the red Master Warning light on a two mile final and I got voice alerts from the Chelton and the engine monitor: "Check Engine" and "Check oil pressure." A check of the oil pressure on the engine monitor showed a big red zero. This went on for a few seconds, then the alarm went off, then it went on and off several times.
Taxiing back on the runway I twice hit the brakes, first while moving fairly slowly and second while moving faster, but holding engine RPM constant (about 900 RM according to the data). Both times the oil pressure sagged momentarily (but not low enough to trigger an alarm).
I downloaded the engine monitor data and sure enough the oil pressure dropped to zero for several seconds, then went up and down to zero several times, total elapsed time of 37 seconds. I downloaded and studied the data for the entire flight and found an oil pressure sag while taxiing out for take off when I braked before the U turn at the end of the runway, but not low enough for an alarm. I also found an oil pressure sag (not low enough to trip the alarm) during deceleration during landing roll out.
I had another engineer in the right seat who watched all of this. We examined the data traces. None of it looked like an electrical intermittent on the oil pressure sensor which is mounted on the fire wall. Our conclusion: we had sucked air.
We check oil pressure at every stop. During a multi-stop long cross country in March, we landed with as little as 5 quarts after some long legs. The engine has about 30 hours on it since overhaul, and is still breaking in, but we have never seen an oil alarm even with some severe test flying (steep departures and descents as well as hard braking during short field practice).
Sump capacity is 8 quarts although the dip stick goes up to 12. We normally run at 6 quarts since prior experience is that more than 6 quarts blows oil out the breather an on to the belly. After shutdown we measured approximately 5 quarts on the dip stick. A day later it still read 5 quarts. (For reference, I found that Lycoming says the absolute minimum is 2 quarts, but Cirrus recommends a minimum of 6 on the IO-550N, same engine. Presumably there is allowance for oil loss during flight with the Cirrus numbers.) I put a piece of welding rod down the dipstick hole, and found the oil to be just shy of 2 inches deep in the pan. The dipstick comes to within about 0.4 inches of the bottom of the pan. We measured the oil pan and then calculated the oil volume based on the area and depth of oil, and came up with about 5.5 quarts. We had also checked the dipstick at the last oil change, and while not perfect, it checked reasonably well with the oil as we added it quart by quart.
Since the oil pressure alarm was on and off for 37 seconds, we pulled the engine for a full inspection. Current guess work suggests something wrong in the oil pump pick up area. The engine is under warranty, so I am driving it up to the shop and we will disassemble it while we are all present and watching.
So, again, what are you IO-550 guys using for normal oil level, and when do you add oil? What oil stories can others tell?