Depends on how the vehicle is used. My wife drives 7 miles to work and usually comes home for lunch. That's 28 miles a day. A 7 mile trip barely gets the engine up to operating temperature in the winter. The oil in that car gets changed every 3000 miles or less.
Your wife's car falls into the classic "severe" driving conditions. Most manufacturers recommend 5000 miles for severe driving conditions. I'd dig out the owner's manual and see what it says. Even in summer, short trips won't allow the engine and the oil to be completely warmed up. If the oil is still cool, it cannot absorb the contaminants that come from internal combustion as efficiently. Changing it every 3000 miles may seem like cheap insurance, but it's very likely just wasting oil, almost all of it imported. Oil is meant to get dirty, and with good air and oil filters, it’s highly unlikely that there will ever be so much dirt in oil that it’ll lose its lubricative properties. The bad thing that happens to oil is when it gets saturated with corrosive (and not lubricative) organic acids (RCOOH) that are byproducts of combustion (like with short-trip severe driving conditions). Generally if the engine is run hot enough for a long enough time these organic acids don’t build up and all is well. But when the motor doesn’t get hot enough or spend enough time at high temperature the acids will build up and the oil will lose its electroconductivity properties. Or more accurately, the oil will be more electroconductive, allowing the corrosives to react electronically with the metals in the engine, causing excessive wear (the same thing that happens when you use the wrong engine coolant or fail to change it often enough).
Even Jiffy Lube is now feeling the pressure to recommend oil change intervals based on manufacturer recommendations for "normal" and "severe" driving. They no longer blanketly recommend 3000 miles between changes. They go the 5000-7500 mile route for severe drivers, but based on manufacturer recommendations, and for 10,000-plus for normal drivers, again depending on oil type and manufacturer recommendations.
My van I average about 6000 miles per oil change.
If it were me, I'd pull a 6000 mile sample next time you change the oil and send it to Blackstone for $25 and see what they say about it.
The problem stems not only from what dad did with the '57 Chevy, back when 3000 mile oil changes were actually necessary, but from the fact that for many people it feels good to get an oil change. If you fill up the car with gas, wash it and change the oil, it runs better. Right? Of course it doesn’t. But that’s the perception.
Contrary to the "conventional wisdom", the most engine wear between oil changes occurs within the first 3000 miles after an oil change. It sounds like a load of crap, I know, but it's true. And if you change your oil every 3000 miles as cheap insurance, you're increasing engine wear rather than decreasing it.
At one time there was an mind-numbingly long PDF file on the net somewhere that detailed exhaustive cooperative studies by several car makers and oil manufacturers, where they were doing things like completely dismantling engines from several different vehicles at various oil change intervals and examining the engine wear under controlled testing conditions. In another study performed by Ford and ConocoPhillips, outlined in SAE Technical Paper 2003-01-3119 ("Antiwear Performance of Low Phosphorus Engine Oils on Tappet Inserts in Motored Sliding Valvetrain Test"), the results showed unambiguously that engine wear decreased with miles on the oil.
There have been many tests performed using real-world oil analysis to either refute or confirm these studies. All of them that I know of have confirmed the conclusions that too frequent oil changes causes additional engine wear. Here
Mobil 1 Test Results is one such study. It also confirms the larger study that showed the importance of adding "top off" oil as needed between changes. Don't wait until you're a quart low to add oil, do it at 1/2 a quart or even 1/4 of a quart. Topping off has a dramatic effect of lengthening the time between oil changes.
When I first got my Sprinter I started a program of engine oil analysis on an almost anal level mainly for the purposes of confirming the Oil Quality Sensor of the Sprinter and in trusting the ASSYST computer in telling me when I needed to change the oil. The oil analysis confirmed precisely what the ASSYST computer was telling me, even though it was contrary to the established beliefs that frequent oil changes was better.
I use Mobile 1 synthetic oil, change the oil filter-only every 5000-7000 miles, and top off the oil level as needed. I get between 15,000 and 17,500 miles between oil changes, as per the ASSYST, sometimes up to 20,000.
My recommendation would be not to change the oil based on conventional wisdom and common convention, but rather based on the manufacturer's recommendations and/or by what an oil analysis tells you the interval needs to be. You can guess all day long, but all you're doing is guessing. And of you guess wrong, you could actually be doing more damage by changing the oil too soon. But more to the point, with every oil change you increase the chances of some of it ending up in a landfill, and you increase oil imports of foreign oil, not to mention wasting money.
If all else fails and you're just not sure, go by taste. When the oil has lost its sweet nutty flavor and bold mouth-feel, it’s time for a change.