Keyword "current". Other than the 7.4l none of those engines existed in 1993. Also comparing current oil recommendations for GM engines has no relevance to a 28 year old Ford engine in my eyes. Ilmor did not produce marine engines until 2002.
Having said that, your testament to the MC 888 5.0 using 5W30 certainly holds merit. 2,000 hours is nothing to sneeze at.
My 80's era Sea Ray Pachanga with a 350 Olds & Berkely squirter ran Castrol 20W50 exclusively but that was in Texas summers.
True, the LSx engines are a more recent arrival, but the point was that the visc recommendation for some of them doesn't vary from their automotive applications.
The 7.4L is probably the most interesting example, spec'ing 15w-50, which is a reasonably broad-spread multi-grade, but obviously synthetic with higher levels of AW additives.
HDEO approvals have a pretty good corollary with marine use. The shear resistance test, and usage profile of these engines aligns quite well, so it's not surprising to see the 6.0L LS-family engine spec a 15w-40 in the above table, and if 15w-40 is appropriate, I'd have no problem using a 5w-40 (which I do in fact use) because it has to pass all the same tests and carry the same approvals.
The historic use of cheap VII polymers in PCMO's (that wouldn't pass the HDEO testing protocols) is the reason those oils have historically been advised against/avoided in marine use. This has only recently (relatively speaking) been changing as OEM approvals have raised the bar. Of course Euro lubes have similar requirements for shear resistance as HDEO's, but it's not practical for OEM's to spec Euro lubes. This is why we the shift was from mono or nearly mono grades to HDEO's for the same family of engines in more recent owner's manuals.
Now, this is from the PCM 2016 manual:
So it seems clear that the manufacturer still does not trust the adequacy of conventional multi-grades at higher temperatures, so again our HDEO is spec'd. We also see specific mention of a synthetic 5w-30 for two specific engines, similar to Ilmor. It would seem the confidence in synthetic 5w-30's is significantly higher (amusingly, that's the same oil we ran in our old Y-block).
Indmar seems to have the broadest selection of older manuals:
Request Test Drive Mail sent successfully. SEND
www.indmar.com
From their 2001 version, they only recommend a 15w-40:
And note that the lineup of engines is primarily old school SBC's, so of similar vintage to the Ford we are currently discussing.
1993 and older appears to be the crossover for their manuals where they spec'd SAE 30. From 1994 to 2002, that changed to 15w-40 across the board.
So, to expand on my earlier recommendation, I'd have no qualms about running a 5w-40 or 15w-40 in the OP's engine or even a synthetic 10w-30 HDEO.
