I've been an A&P/I.A. for about 14 years, but I have never had the time to read through forums until now. This is interesting and I want to add some additional thoughts.
I agree with Ted about everything he has stated so far as to preserving engines, oil viscosity, pre-heating, and moisture control. However, there is one more thing that a LOT of owners, A&P's, commercial operators, etc. miss when considering engine oil and maintaining an engine for the "Long Haul" to overhaul.
Most owners (not all) will simply add oil to a piston engine without considering WHY they are adding oil, and that is an issue in my opinion. Simply put, engine oil should be considered a "Component" of the entire engine, such as a magneto, carburetor/fuel servo, etc. When checking engine oil if it appears to be grey/black and clearly used (in my experience this begins to occur between 25 and 35 hours after oil changes), the oil is being "used" by the engine in some manner.
Question this.... WHY is the oil being used? HMMMMMM...... maybe you haven't considered this? Think of this exercise like a magneto. When magneto drops become excessive you repair the magneto.... when engine oil burn is increasing perhaps the oil has lost some of its properties from new. The property of the oil DOES change over time (both calendar and hourly working time), that is why we change oil.
Lycoming recommendations are a 50 hour oil change. However, that is a recommendation and should be not be considered the maintenance gospel. After an oil change, it is common to see oil "Consumption" until the oil level stabilizes (with a couple of hours). This consumption is typically (Not always) blown over out of the crankcase vent. In my experience Lycoming engines like to run at the lower side of the oil levels, but will run about 25 to 35 hours depending on the overall engine health before they begin to ACTUALLY burn oil. This isn't simply pumping oil overboard, but can be caused by the oil losing its viscosity. Basically, between 25 and 35 hours, the becomes less effective.
I HIGHLY recommend to my clients that they change their oil any time when they see 1) Dark coloration of the oil AND 2) When they need to add oil. I recommend that they drain the old oil and put fresh oil back into the engine. This maintains the properties of the oil and provides good lubricating properties throughout the engine life.
I have used this practice in the past at a helicopter flight school. We operated 12 helicopters total and flew an average of 600 hours per helicopter per year. In 4 years and a combined total of approximately 28,000 flight hours later I never changed a single cylinder; never had a stuck valve; and EVERY engine made TBO. I'm not contributing that ALL to the practices with oil changes, but I FEEL (cannot verify) that it certainly helped.
I now this is an old thread, but I'm new here and wanted to put my two cents in somewhere. Fly safe.... BUY OIL!