Joe, for the 3L merc 4cyl, you can use the AC delco PF35 filter as a replacement. The pf35 is a longer filter, I don't remember what the AC # was for the same length replacement.
I aquired a 2002 bayliner 19' end of last summer, it has the 4cyl 3L merc i/o. I did oil analysis on the factory oil, merc 25w40, using dyson analysis. I think you might still be able to find the report in the UOA section. The oil sheared right into a 30wt oil. But for what it's worth I do a lot of slalom skiing with the boat so the motor runs at 4k most of the time and it is also new so the shearing may also be attributed to break-in somewhat. I think I put somewheres around 10 hours on the engine when I got the boat before changing the oil, and I figure the previous owner put no more than 5 hrs on it from when he got it brand new.
I have a gallon of merc 25w40 my neighbor gave me so I hope to use that for part of this upcoming season then do another analysis on it.
On my 24' formula, 1976, I have a factory new ford 351 I dropped in back in 1993. I ran redline 10w40 synthetic in it last season from June to December here in CT. Oil analysis showed it held up excellent, low wear, stayed in viscosity, and had a good TBM report. That motor spends most of it's time at 3k rpms but sees occasional 3500-3700 from skiing, until I got the bayliner.