What's your oil of choice for you Mopar? I usually use Valvoline Conventional 5W-30, just as my grandpa did with the car, and a Fram spin-on type oil filter. It was converted years ago, but I still have the original assembly.
With that being said...do you think it would be OK to use a bottle of STP Oil Additive? Or maybe Lucas? I have used STP several times, on other cars and I love it. One of which, was my girlfriend's/her dad's 1995 Crown Victoria that had a bad oil leak, and it really slowed it down. Just asking since it's an old motor. It was rebuilt in the late 1980's.