Advice on switching oil

I have a 1985 300D with 189,000 miles. I currently use the Rotella non-synthetic oil available at Wal-mart for about $6.25/gallon. I change my oil and filter every 3,000 miles. My car does not burn any oil. I have no trouble starting the car in the winter (the coldest it gets in Richmond, VA is about the 20s). I recently found a CAT dealer which sells the Mobil Super 1300 15W 40 full synthetic oil for $11.30/gallon.

I know that generally synthetic oil is better than regular oil but I don't want to start a new debate on whether this is true for an older engine. However, because of the relatively low cost of switching to the Mobil, I am giving it some consideration ($12.50 per oil change vs. $22.60 per oil change) Any thoughts? Also, would 15W 40 be the weight I need?

Thanks for any advice.

Scott