All ICE engines accumulate fuel in the oil, some more than others. That's pretty much normal, regardless of the type of driving, in city, highway, etc. As long as the oil contamination is within "normal" level, that does not pose any issue to the engine. For the RL's V6 engine, the normal is under 2% of fuel in the oil, as stated by Blackstone.
I drive my RL 98% of the times in city traffic, doing 3 - 6 miles round trip with one or more stops, the engine barely reaches operating temperature at times. The first oil change for my RL had been over a month ego, with 3K miles on the odometer. Blackstone report shows:
Even under these condition, the fuel content, viscosity, etc., are within the acceptable limits, even for city driving pretty much all the times. Just change the oil yearly, as specified by the manual/MM.
As for the mileage with mostly city driving, it's 12 - 13 MPG in the winter and 17 - 18 MPG in the summer. During my vacation trip last summer, mainly highway driving, the mileage was around 26 - 27 MPG. It's pretty much the same as my Maverick with EB2.0 engine had been under the same driving conditions. That's not too bad for a N/A V6, 3.5L...