Hey folks, it's about time to change my oil and I wanted to know if I should use a synthetic oil or not?
What weight?
The car has 165000 miles on it and it runs so sweet.

I can feel a flame war!!
 

I think the best thing for you to use would be Mobil 1, synthetic. You should do a search on this. There have been MANY topics covering this. You will learn a lot of valuable information and be able to decide for yourself which oil is best for you.
What kind of climate are you in? I use 20W 50, but it never goes below 40 F here! If it goes below 23 F, then you should use something lower than 20W. Good luck and don't forget the search!

In your owners manual there is an extensive list of recommended oils. I'll fax mine to you if you don't have it. I have a 91 560 SEL 97K miles. It "uses" more w/ Mobil 1 0w-30 than Penzoil 5w-30( lazy in the winter, Jiffylubed ).

There's a big oil thread going on ...again. I change Mobil 1
every eight weeks, not that I need to, Just because I can.

Hope your's stay's tight!!
Jim

What weight to use is based on expected outside temp. I have seen some owner's manuals saying not to use 5W-30 if it's 60 F or higher and not to use it on long highway trips during the summer months. I don't remember what car it was for, but this just goes to show it is based on the car and climate. For my car, the owner's manual say a good year-round oil would be 10W-40 or 50. It's good from -4F to 86F. Currently I have 15W-40 in it, which per the graph in the manual, is good from 32F-86F.
Now as far as the old syn. vs. dino. question, like Jim said there is currently thread on this subject. Among gasoline owners, Mobil 1 seem to be the most popular for synthetic oil. For diesel owners using synthetic, it's either Mobil 1, or Mobil 1 Delvac.