With the exception of the UPF44, the ACDelco Ultraguards are no more. That short-lived filter line was absolutely the best oil filter on the market--and still would be, today. Unfortunately, GM (typically) had a great product but failed to market it properly. Sales were slow. GM was in a short-term profit mode so Ultraguard got killed after a year and half.
The K&N Performance Gold is a pretty good filter but I think, right now, the Mobil 1 filter is about the best oil filter which is mass-marketed. I'm pretty sure it's made by Champion Filter (not to be confused by Champion spark plugs) which is an outstanding mfg. of filters and is often used by big companies to "private label" relatively small runs of a product. Champion makes it to Mobil Oil's specs.
I use the M1 filter on my ZR1. In fact, I'd use them on my other cars but an Ultraguard (which was even better) was available for each of them, where one was not avail. for the LT5, so shortly after GM announced it was going to discontinue Ultraguard, I bought a large supply of PF52s and PF1218s.
Mobil 1 filters are just a bit pricey--probably because Mobil uses the "Mobil 1" brand on them--but I think they're the best choice for any high-performance engine.
