When to change synthetic oil?
 

I've had pennzoil full synthetic in my 02 for about almost a year now and wondering when to change. On google I keep getting mixed opinions.

I know official sites will say more often to keep business but other sites like; http://www.edmunds.com/car-care/stop...-your-oil.html say to do it ever 10 to 15k but they might be refering to newer cars.

I know it differs on engine size too so its hard to get a direct answer. Anyone know?

Well i'd change it now, since its been there for a year. In an older pickup probably every 3-4k or so

I change every 5000 miles or when its dirty. Synthetic change

I agree at 5000 miles doing a full oil change. But that said, with full synthetics, you can safely change your filter at 5k the first time and change oil and filter at 10k, or so they say on one of the oil companies sites I was on. They did strongly recommend using a high quality filter (promoting their own of course), and there are few better than the Motorcraft FL820S for our vehicles. As long as it isn't a Fram (or another filter of lesser quality pleating), you are good to go. You can never go wrong with a Motorcraft filter on a Ford. JMO, of course.

Many cars have electronic systems monitoring when oil should be changed. They consider average operating temperature, mileage, fuel quantity used, number of engine starts and stops, etc. to determine how hard the oil has been working. Oil needs to be replaced when it becomes contaminated with water, dirt, carbon, sulfur, fuel, etc. and/or when additives are depleted, and/ or when lubricity is reduced. Changing oil too often costs money but too late is worse of course.
Cars with electronic monitoring systems using full synthetics typically delay oil changes to 15,000 to 18,000 miles. This is for mostly highway miles where the load is low and water is boiled off. For lots of city driving this can be down to 10,000 miles. These are conservative mileages and keep the engine protected based on millions of engine-miles as tested by the OEM.
The oil filter should be replaced at every oil change mainly to avoid the liter of old oil that would contaminate the fresh oil. It would be highly unlikely that an oil filter is clogged enough to trigger the bypass in even 20,000 miles.
Based on all of the above, synthetic oil/filter changes at 10,000 (mostly short trips) to 20,000 miles (mostly highway) would be safe.
Replacing oil when it looks dirty is good if the mileage/ load is unknown (just bought the vehicle) but just like we can smell gas in very low concentrations, oil gets darker with minute amounts of tiny particles, so "not new" looking oil is probably too early, but only a question of $$.
In my experience, two areas of "oil change" caution: switching to synthetic oil often leads older engines to leak when they didn't with non-synthetics; and never use an engine oil flush with older engines unless you plan to drop the pan and physically remove the loosened gunk.

it will depend on driving style.
Lots of highway mileage, you could go 10,000+ miles, lots of city, off-road, traffic, extreme heat/cold or short drives could be as little as 3000 miles (even with synthetic).

Only way to know for sure is to pay to have a used oil analysis, which is almost the cost of a synthetic oil change.