I'll start with the basics.
Oil gauge fluctuations are part of the MB character. Unlike other vehicles where the oil and temp gauges have the needle hold steady at some "automotive karma" position, the MB gauges actually provide useful info. The numbers on the gauges indicate pressure in units known as "bar" (a bit different from atmospheric pressure, but close). You should typically see the needle pegged at "3" after starting, and when driving under load. Otherwise, the needle goes down a bit when idling (between 1 and 3 depending on engine configuration and condition).
I don't think your problem is oil related. A major leak would set off the oil level light on your dash, and pressure seems to be fine from what you describe.
If you were experiencing an overheating condition as a result of oil starvation somewhere, your coolant temp would still register the anomaly, and your engine performance would increasingly worsen as you drive.
Some other info would be needed...does your car idle smoothly? Does it start fine warm or cold, of differently depending on whether your engine is warm or not? Do you hear the whirring sound near the rear passenger tire? (That's your fuel pump.)
Also check your OVP (overvoltage protection relay, LOTS of threads on this), especially the fuse.
My thought without seeing your car? The timing chain. When's the last time the timing chain was replaced? If it's stretched, it may have jumped a tooth or two causing the timing to change a couple of degrees...perhaps enough to make starting difficult, and hamper overall performance.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car