Mike,
Summary: My educated guess - Go with the 5000, but don't go too much longer than a year.
Being in a similar situation, I discussed this very issue with an engineer friend at Exxon/Mobil some years back. I was most curious because at the introduction, Mobil was saying that passcar applications could run 25K miles and at 25K or one year, the oil filter should be replaced and the lube oil brought back to level.
He remarked on my good memory and said that they had to revise this because of the accumulation of moisture and combustion by-products in the lube oil that caused corrosion.
Remember that Mobil 1 was introduced many years ago. Engines run much cleaner now than they did then. I expect that the combustion by-products are cleaner and there is much less of them with the advent of round cylinder bores and other such things.
So, if you don't get to 5000 in a calendar year, just don't go too much past that. I would not let it go through two winters.
Matt
