In many cases they put a time limit on oil life in addition to a mileage limit because if you don't use the vehicle much, it could be subject to excessive cold starts with very little accumulated mileage to warm the engine (oil) up fully. As previously mentioned, moisture and acids build up in the oil over time. When you have a lot of cold starts, unburnt fuel can also build up in the oil because of the temporary rich condition at cold start. That reduces the oil viscosity. Before electronic fuel injection became popular, that was a much bigger problem. Now, not so much but, many manufacturers still put a time limit on oil life. Manufacturers have no idea how their customers are going to use their product so in many cases IMO it just CYA.
When I was a kid, the oil in grandmas car was always overfull and looked like water. She only drove the 390 cubic inch 4 barrel carbureted monster to church on Sunday and to the local grocery store.
Chum lee
