My mechanic explained it to me this way: In order for oil to get hot enough to get all the moisture out of it, the engine needs to run at least 45 minutes or so. Shorter trips won't do the trick. So if you take lots of short trips, then moisture builds up in the oil (so I guess oil and water do mix). This is the reason that lots of short trips is considered extreme driving conditions. Every vehicle owners manual states this. Every manual also states that the oil should be changed every 90 days for extreme driving conditions. So, according to my mechanic (and he has over 30 years experience) if you drive lots of miles for extended periods of time, use synthetic oil and change it every 7,000 miles or so. If you drive short trips or infrequently, change the oil every 90 days and use regular oil. For short trip driving, synthetic won't do you any good and costs twice as much. The debate as to how often to change the oil, and whether or not to use synthetic can rage on and on. I wanted to offer the opinion of a professional mechanic. I like to take the advice of experts instead of relying on an opinion poll.
