There are some gray areas Tony, all I can offer is what I've seen. I can tell you I've never seen a wiped cam on a mercruiser running mercruiser mineral oil. I've seen the 3.0 with over 6000 hours and still running like a top, on mineral oil this is not uncommon.
Here's a little background on what flat tappet cam manufacturers recommend for cam break in:
http://www.cranecams.com/pdf-tech-tips/cam_failure811.pdf
As far as I know, the Mercruiser synthetic oil was recommended for roller cam motors fitted with catalytic converters, not catless flat tappet motors. I may not be completely correct about that but the reason for removing metals from the oil was to protect cat converters from being contaminated.
I have seen several flat tappet cams lost on seasonal use engines filled with synthetic oil, but can't say I've noticed that for mineral oil. Okay, maybe that's a strange observation I cannot prove but even synthetic oil manufacturers have documented they often recommend their mineral based product in cases where the engine is not regularly run b/c the synthetic oil runs off the cold surfaces faster and they noticed corrosion issues due to this.
So being me, I put two and two together with my experience and conclude that for seasonal use in a flat tappet motor I prefer the old mineral oils that we ran for thousands of hours and never had camshaft issues. This is MY conclusion, the 3.0 is a bulletproof motor when loaded with mineral oil, they just go and go forever. Try and wear one out, I don't think you can unless you run it over 6000 hours maybe and probably more than that. The stern drive will wear out first.
Nothing wrong with the mineral oil in the 3.0, it's proven to work and protect the cam. Run the synthetic if you want, but mineral is MY recommendation for this motor.
You asked my opinion, I gave you my best answer based on what I think I know.
I have no problem with using Rotella oil in this motor either, just that I have not used it in gasoline marine engines before and I have to wonder what's been done to Rotella lately since newer diesels are now running cat converters?
I guess if you really want to be safe you could send your oil off for analysis, they can tell you how much copper/zinc/sulfur and whatever other traces are in the oil before and after being run a season but to me this is pointless for us guys that might rack up 10's of hours between oil changes as long as we're running something made for a flat-tappet motor.
There will always be off-road engine oils available for us hopefully, like the Valvoline racing oil and I'm pretty sure the straight weight 40 oils still contain the additive package just as the mercruiser mineral oil does I'm sure. I don't know about the rotella stuff, wonder if the diesel oils aren't already metal free since the cats are being used on the trucks now those are high in detergent I think and tend to turn black quickly for some reason so I've not used them in gasoline motors.
