You should be using synthetic. Mobil 1 is good, there are better, but the real question is are they worth the extra money over Mobil 1. If you are/were racing the car I'd say no oil is too good. For normal driving, the regular dino oil is plenty good. You don't have to use synthetic at this point because your warranty is expired. I would say that since you have kept the oil changed very frequently that you haven't done any harm by running regular Castrol oil. I wouldn't worry too much about the oil. Switching back to Mobil 1 isn't a bad idea but if you aren't racing I wouldn't sweat it.
As for the filters, there really isn't a bad filter, just like if you keep fresh oil in it there isn't a bad oil. The Fram's performance is lower quality compared to a Mobil one, an AC/Delco or a K&N filter. Have you done any damage by using a Fram and changing it at 3,000 miles, my guess is definately not.
If you want to use extended oil life and filter life you need the best. The LS1 with an oil life monitor using synthetic and normal driving is probably going to need to be changed about every 9,000 miles. In that case, or in the case of racing the car, you absolutely need a top notch filter and a top quality synthetic oil.
I'm a maintenance freak. I change things way too early because I never want to have a failure due to lack of maintenance.
I change my brake fluid every 4 years, that might be something you want to do now. Your calipers will last forever.
I think the 04 has an external Fuel filter, you might ask an 04 owner, if it does change it. You should change it every 30,000 miles to keep the fuel pump happy.
I would also look at changing power steering fluid, gear lube in the differential, and the A4 fluid. I would change all of those fluids every 3 years no matter what or 48,000 miles. Time matters more than mileage. The owners manual will tell you the weights of the fluids. Just make sure that the fluid meets the API reccomendation for the car and you will have the right brand. Again if you are racing look for the best, an Amsoil, Royal Purple, Redline or such is the only thing to use in extreme situations.
Every 5 years in the south, and 7 years in the north replace your serpentine belt. Also every 5 years replace your coolant. I replace my headlights and taillights every 5 years too, but I definately drive more than you. The battery is another thing that time matters on. I replace the original battery at 4 years old, which is right where your battery is now. Buy an Optima and you'll be good for 5-6 years.
I've gotten 300,000 miles out of two Cavalier company cars, and neither one broke down or left me stranded. With 5,000 mile oil changes they were't using more than a quart of oil between changes either. I would much rather pay for a little maintenance than end up in a parking lot looking for a jump start or on the side of the road calling a tow truck.