I actually have a very similar case with my 2003 Dakota 4.7L. I've been driving less than 300 miles a month for the last year, and I was concerned about fuel dilution since I'm really just driving 7 miles one way to and from work.
I had 8 months and 1,600 miles on the Pennzoil Platinum 5w30 that I just sent in for analysis. They said it looked great (just a trace of fuel in the oil) and that it'd be fine to go for a full year. That was with Pennzoil Platinum 5w30 and a Pure One oversized filter.
Blackstone's exact words as follows:
"Not to worry. Despite all the city driving, you're not getting any excessive fuel in the oil. You've got
just a trace here, which is perfectly acceptable. We consider anything up to 2.0% (and often more than that)
to be normal, so as long as you're around that level, we won't think the city driving is a problem. Wear
improved a lot, but that's because you changed this oil at just 1538 miles. The last sample had 6100 miles
on it. You don't need to stay this conservative, if you don't want. Yearly (or possibly even less often) oil
changes sholud be fine."