AdRock, I mean this to be constructive, not criticizing.
The first thing you need to do is get over the 3K oil change. I know it's hard to do as that is the way I was raised and I did it for many years, with both conventional oil and synthetics. It is the 21st century, half way through 2009, and there is not an API approved oil on the planet that cannot do 5K oil changes.
Now, if you are going to use a quality synthetic like Amsoil this is what I would do. Get the Amsoil of your choice and get the Amsoil EaO filter for your applications, change the oil on both vehicles and don't worry about it for at least 7.5K. I know it will be hard, but it can be done.
Now, as for as filtering used oil. You might run it through a fine screen or a paper filter, and you might filter out some dirt particles, but you cannot filter out the contaminants produced from combustion process. And even at 3K those contaminants are in there.
I guess the first question we should have asked is what year models do you and your wife drive, what is the mileage on both vehicles, and what kind of conditions are these cars driven, i.e. mostly highway, city, etc.