I've run the 20w-50 syntec in my boat- a 1993 factory marine Ford 351w HO. It has the gt40 heads, and a holley 4160 4-barrel. 285hp.
I think I ran that oil around 1999'ish, right after I got my '99 SS. Engine now has around 600 hours I think, still runs great, doesn't miss a beat. It usually runs 2800 rpms, but if I need to get to where the fish are then 3500 rpms. Never did oil analysis on it though (the castrol). I didn't start doing UOA's till around 2002-03.
I'm still up in the air as to what oil to run in my 2002 bayliner, it has a 3.0L 4-cylinder merc. I ran redline 10w-40 in it last season, and for the past 2 seasons in the 351w. UOA's were posted, nothing spectacular. Suggestions were to use a 50wt oil in the 4-cylinder to help reduce lead #'s (bearing wear) since that motor always runs 4000+ rpms (I ski). I winterized both motors with delvac1300 15w-40, and if I don't decide to run the delvac in the 4-cyl for next season, then I may try the syntec 5w-50 or 20w-50.
For what it's worth, I've run just about every oil in the 351w over the years, including a dose of slick50 ptfe, probably around 1995. I've also used RESTORE, also around that time. I started out with castrol 30HD dino for break-in, then used various dino's (mostly castrol), then switched to the synthetics around year 2000. I've done numerous seasons early on with castrol 30HD and castrol 20w-50 dino. Although I changed the oil twice per season then, the oil never really turned dark and was always hard to see on the dipstick because it stayed so clean. When UI switched to 5w-50 and 20w-50 syntec, it kept very good oil pressure (40+) at idle after running a long time. Before on the dino 30 weights, I would see around 20-25psi oil pressure at hot idle after running hard. Sorry though, never heard of UOA back then. I've run 5w-30 and 10w-30 synthetics (mobil, pennzoil, maybe castrol) during the september to december months and I would have 10-20psi oil pressure at hot idle after running hard for a while. I also used Fram (PH8A) filters early on, if not the motorcraft FL-1A. Since 1999 or so, I've been using AC-Delco. I've also used hasting's filters, and didn't notice any difference with those.
Two seasons ago, I had an exhaust riser gasket fail and leaked water into two cylinders over night. Went to start it one morning and a cylinder fired and compressed the water filled cylinders and blew out the head gasket. I pulled the heads, was around 450 engine hrs, to fix. Fortunately no bent rods. I figured have the heads completely reworked since they're off. So with all new valve seats, I reassembled everything and compression tested. Pressures tested between high 130's to 150's from what I can remember. The motor is 8:1 I believe, and specifically designed to run on 87 unleaded. So after all the different oils, I concluded the rings were still 100%.