I would get the oil pressure regulator checked right away. Hoping it is OK because you have no leaks is like hoping you are not having a heart attack or stroke because you are not bleeding. A loss of oil flow can be fatal to the engine...

As far as mileage is concerned, I noticed mine had dropped considerably as well. Prior posts about winter gas blends are accurate. But be sure to chck the basics first then move into the harder areas. How is tire pressure? Since it just got colder, you may be losing relative tire pressure. How is the condition of your plugs, filters, oil level, etc?

Just don't forget the basics. I got my '99 Dakota back to within 1 mpg just by doing that...

 

I've had three Dakotas and each have had substantial mileage drops on them. My 91 4 cyl. dropped mileage after I towed an overloaded u-haul trailer about 450 miles, I figured that one was my own fault (it still got high teens low 20's even after the drop). My second was a 97 3.9L standard cab. About 35-40,000 miles, the mileage dropped from mid 20's to low-mid teens. I never really looked into that one because I traded the truck for a 2000 3.9L quad-cab. It had already had 60000 miles on it and the mileage was down around the low-mid teens just like the 97. I've tried the tune-up and maybe gained 1-2 MPG. I've considered swapping out the O2 sensors, but at $80 per sensor, that's a lot of money to put out on a "what if." If anyone knows how to check an O2 sensor at home, please pass that along. Of course the real problem probably is that each one of these trucks has had to haul my big ol' fat butt around.

 

Well, I may have found some of the oil issue. I took the air hat off this past weekend just looking around, and it looked like there was oil in the air hat, inside the intake, and in the crankcase breather tube. One of the blades or fins or whatever they're called on the throttle body was stained a light brown. So from everything I've read it's looking like my plenum gasket needs replaced.

 

an easy way to check is to pull off the breather hose and put your hand over it... if there is a vacuum then your plenum is leaking. normally it should build up pressure that you can feel blow out when you pull your hand off the hose.