I'd say yes to the fuel additive, it's good routine maintenance to do so. Oil additive only if you have problems. And I'd never recommend a radiator additive. If you have problems with the cooling system, replace the necessary components instead of masking their failures with an additive. If the cooling system does fine, I'd leave it alone.

 

Ok, you guys do not like to replace what isn't broken. Yes, but what if you want hassle-free breakdown-free safe driving for the next 100k? Which of these throws a code before breaking down on you? I think I want to at least replace the CPK b/c that can result in a no start condition. And, it seems like the most common failure in this list of 5 sensors. If I replace, I will keep my old on as a spare. I also ONLY buy OEM sensors, so a bad one is a low risk. Thoughts?

 

The suggestions everyone else has given I totally agree with.

Oil is a very touchy subject and has been covered a million times, sometimes twice a week. Shell Rotella is a very popular oil here, it comes in a variety of weights and grades. The best weight for a 4.0 in good shape is 0W30, seeing as you're in Texas you can probably get away with 10W30 and 15W40 on a regular basis unless you live in a part that actually gets cold.

FRAM filters are awful and the OEM makes the same design with other labels like ACDelco, Quaker State, etc. Pay close attention to the price point of your filter. If it looks cheap enough that you think "man, the only way they could make a profit is if that was made of cardboard!" then you're probably right. NAPA Gold/WIX, Purolator, Motorcraft, Mopar, Mobil1 are great filters with varying price points.

Factory spec spark plugs for a 1998 should be Champion RC12ECC, however Chrysler has superseded these with NGK ZFR5N in some cases, so they are fine to run. IMO the double-platinum plugs (these are pretty much the only "fancy" ones that work reliably on the 4.0) are a waste of money... the engines are old enough that you're probably going to have to replace them earlier simply due to fouling and deposits.

I'm a fan of the NGK spark plug wires, part number RC-CRX026. The wires are labelled! Up here I see them on sale every few months for 20% off.

With that attitude then, the only one really worth it would be a new CKP from the dealer. The other ones are not likely to fail (most IAC, TPS, and MAP problems are a result of being dirty) and if they do won't strand you like the CKP will.

 

Thanks salad, that's what I wanted to know.
I will probably replace the CPK for $75 OEM as a precaution.
1998 with 125k, the CPK will probably go sooner or later anyway.

The rest will just throw a code and have some rough idle, but won't strand me on the side of the highway.
I will replace those as they fail.