I was wondering if the type of motor oil used affect gas mileage (or I should say Diesel mileage)?

Until recently, I have been using 20w50 dino oil in my 84 300SD, but switched to Shell Rotella T (not synthetic) around Thanksgiving. I drive a lot of Houston freeways, and used to get 21-22 mpg. However, my recent mileage is more like 23-24 mpg. Does this have anything to do with the oil, or just because of the cooler weather and less A/C use?

All of those things you mentioned are legitimate variables in MPG. Tire pressure can also affect gas mileage. As they say in all forms of troubleshooting: Never underestimate the power of coincidence!

You gas mileage variances could be the result of:
Tire pressure
Weater
Humidity level
temperature
wind
Driver's inconsistances.
Variations in fuel used.
Traffic congestion
Speed
Road surface
and possibly 10 other variables.
MPG calculations on the road showing variances of +-20% is totally irrelevant.
jackD

And didn't we all fall for that old Shell commercial where the car tears through the paper banner marking where the other gas had run out!

Yeah! Super Shell with platformate. How do we remember those things.