Over the years I have typically run a quart of MMO with 5 quarts of oil, because most oil change specials are 5 quarts and a filter for XX.XX price.
I still run a quart of MMO, with 5+ quarts of oil. Both my XJ and MJ are Renix, and I found a larger metric-thread filter. Which adds approx .5 quarts.
At least once a year I use Berryman's B-12 Chemtool in the crankcase. Basically the same as Seafoam, but half the price, at least a day or so if not a few hundred miles before oil change. Oldtimers swear buy using it for the whole oil change interval, but it is a solvent-based product. I have done it, with no ill-effects, but YMMV.
I keep at least one bottle of MMO in all my vehicles, as it is a universal fluid for oil-based items. DO NOT RUN IN YOUR COOLING SYSTEM.
Once a month I use either MMO, or B-12 in my fuel tank. Once a year or so, I use B-12 through the BB vac tube to help clean out the top end and such.