I've always used Mobil 1 or T6 Syn in my MAN CR1550's. Pulled them apart for a major today. 5 years on the after coolers an 30 months on the heat exchanger. I think the after coolers are pretty amazing that they are so clean. One of the benefits of running synthetic oil. Obviously, the oil doesn't matter to the heat exchangers but I'm surprised how clean they are too. About 4 years on the coolant and no build up from it either.
Not that many hours per year. Maybe 100 or so per year and almost all of it at 1850 rpm I had scoped the AC’s about 2 years ago and delayed the service based on that and the intake temps. The HE’s had new O rings 2 years ago and we’re very clean then. So far the tech is having good luck with no broken bolts. I don’t anticipate any cooling improvement but it will be good to have everything cleaned, pressure tested and put back together with new gaskets.
That's about average so I've heard I do about 100 also. Some engines just sit for weeks, and I suspect that has some negative affects on the internals?