The venerable 4.6L 2v mod motor is VERY robust and does not really care what oil/filter you use. If you cannot get 300-400k miles AT A MINIMUM from that engine, you're not even trying hard. Being a fairly old design, the 2v in particular, it does not have a bunch of tight clearances or VVT, etc to worry about. Whatever got past those MC820 filters likely did little harm. These engines run literally hundreds of thousands of miles with just basic service in horridly abusive (taxi service for example) treatment. Any BITOGer need not worry about the lifespan of this engine.
That being said, there's a reasonable cautionary concern for switching away from the 820 to a different decent brand. Your choice of NG (Wix) is a fine one; there are many others.
I had two MGMs with that same motor. Both are still running fine, although no longer in my direct control (one to my son with 140k miles and one to his roommate with 260k miles). Both those engines got 10k mile OFCIs with the most basic of lubes (RK dino 5w-20) and yet turned in perfectly desirable wear data. Those engine do NOT need syns or 3k mile OFCIs to last hundreds of thousands of miles. My UOA data and experiments are posted in the UOAs section of this site. If you want to waste money on OFCIs, that's totally OK with me, but it's not necessary by any stretch of the imagination. The 2v series of mod motors (4.6L, 5.4L, 6.8L) simply do not give a darn about the lube/filter you use as long as they are of any decent quality.
