Well im the proud owner of an f150 3 valve 5.4 and all of its issues.
When I bought the truck 30k miles ago it had some pretty serious sludge/varnish problems to go along with all the rattling and phaser problems.
I have used MMO a lot and Kreen a couple times. Its pretty clean now and the rattling is much better.
So long story short I only run synthetic oils now and I usually add a pint of MMO to my oil at 3k and run it to 5k every other oil change. Its been working great. The varnish is still dissipating and it hasn’t started rattling again. To this day though when ever I finish a MMO run I still find a little bit of hard carbon specs in my oil filter.
So to the point: I regularly use MMO with synthetic oils like Pennzoil or valvoline at a ratio of around 12% for 5k ocis without issues.
I also like to run it at the last 1000 miles of an extended drain with synthetic oils. And I have noticed nothing but clean engines.