don't know if anyone ells here will agree with me on this one but if you want to go with a better quality filter i would just go with the filter that goes with the 3800 you have... i would go with a nice quality filter, they have filters that do a nice job, and one of them is NOT Fram in my opinion, many people use them extensively beacuse the are cheep and easy too get but there is a reason for that, FRAM sells beacuse of there cheep price and extensive advertising, i usually get Napa or Wix filter, thats pretty much what a Napa filter is, a Wix filter with the napa logo slapped on... they are a nice quality built filter and only a couple bucks more than a FRAM...

if you have done a LIM gasket change my suggestion is too do a quick filter swap after a couple hundred miles just the filter... just jack up your car, take the old oil filter off, take a new oil filter and put it in just like you are doing a regular oil change, start your car let the oil run through and prime after you put the filter on, check your oil level and fill as necessary, i have done a few of these intake manifold gaskets and this is what i was always told to do, most oils have detergents in them and will clean out the system...

as for oil flush additives, i would suggest not using them a lot of times i have found they can brake loose sludge and it can get caught up in your oil pickup and thats not good, the detergents in the oil are more gentle and slowly clean out the engine, let them do there job.