I was a lubrication engineer for two oil companies for 24 years. The best way to determine oil change intervals is to submit a sample to an oil analysis lab on a periodic basis. As an independent consumer you would probably pay over $25 for this analysis. Half the cost of an oil change for sythetic.
All of the above posts are valid, but just consider that anything can happen over time, such as a coolant leak, which is often catastrophic and an oil analysis may not predict. With a proven synthetic like Mobil 1, oxidation is usually not a factor. It has an excellent additive package which is not normally depleted.
The benefits of sythetics are numerous, even at 4 to 5 times the cost. In cold climates the oil feeds the upper engine more rapidly than mineral based oils. In hot climates oil degradation is much less than its mineral based counterparts.
In my opinion, miles, and how they are driven, are more important than time when determining an oil change. I put 287,000 miles on an 87 pathfinder, 3.0L, used Mobil 1, and changed oil every 20,000 miles. Average mileage was 3500 miles per month. This was the optimum oil change interval when contaminants and wear metals started to approach oil change limits. I never had a physical oil condition, such as viscosity increase/decrease, oxidation, contamination, or additive concerns. I submitted an oil sample about every two months, 6-7000miles.
Filters are also a factor. Buy the best you can. They can deteriorate under high viscosity loads, high contaminant loads, etc. Once you blow a filter and create a bypass condition, all bets are off. You will notice decreased oil pressure if you have a pressure gauge.
Hope this helps. It is hard to make a general recommendation due to the variety of conditions an oil is susceptible to experiencing.
The crankcase is not sealed, far from it actually.
On the other hand it sounds like if you just drive your cars more this wouldn't be an issue. go ahead and put 15k a year on the car then there wont' be any question about why or when to change the oil.
3000 miles! That is 1960's advice. Jiffy Lube will still tell you that for obvious reasons. With today's oils and engine technology you can go much longer. Your car (and most) have an oil life monitor that will tell you when to change oil based on your driving conditions. It is very accurate and has a 100% safety margin built into it. Pre 2000's where capped at 7500 miles and the 2000+ is capped at 12,500. You can trust it. Do a search for OLM or Oil Life Monitor and read up. 3000 mile changes are a waste of time, money and oil.