Anyone that substitutes a good regular maintainence schedule for those miracle cure additives that they hawk on television is only kidding themselves.
There is no one magic bullet brand of oil or additive that can be added to the engine, transmission, rear end, or gas tank that is going to make up for a poorly maintained vehicle.
Look at it like this. If a fuel injection cleaner says all you have to do is pour it into the tank and it will clean your fuel injectors, what would prevent it from picking up all the sludge and dirt off the bottom the tank, out of the fuel filter and out of the fuel lines and running it through the fuel rail and into the fuel injectors - which would cause more problems then it would solve.
The same is true with oil. You want to use the best oil for the application.
At times I will use Pennzoil 5w30 in the winter and 10W40 in the summer.
I change my oil every 3,000 miles and the oil is usually dirty when I change it.
The purpose of the oil is to carry away all the dirt and the sludge and the carbon deposits and move all that crap into the oil filter. The oil filter by the way is over engineered to last 15,000 miles. I always try to substitute the stock oil filter for one that is longer then the stock unit that holds more oil.
I also substitute a bypass oil filter when the mileage gets higher because that not only traps more crud in the filter, it also holds some back pressure against the engine when you shut it off to promote more oil pressure sooner when you start it up when the engine is cold. So in your case, changing the motor oil often and using a high mileage oil filter is a good thing.
I would also change the spark plugs if you have not done it already and I would change the fluid in the transfercase and also the fluid and filter in the automatic transmission. I would forget about performance upgrades and all the crap and stick to the basic's. There is no shift kits anymore, that stuff is all done with power tuners or a Tech II.
Change the fluids in both axles and inspect things like the drive shafts and the universals and the seals on the ends of the axles. Now is the time to repair them while they are leaking and not wait until a bearing failure to do so.
Take apart the disc brake caliper mounts and clean all surfaces and lubricate the pins on the rear mounts and replace any seals that are torn or wore out.
Replace worn rotors and brake pads as necessary to maintain good brakes.
Lubricate all moving parts. That includes door hardware and seat tracks and hinges.
