JohnnyGTO, you've got it. and no, no very significant increase in mpg w/synthetic. But it may help clean out your engine from time to time, or it may be totally unnecessary.
To give you an idea, I have a stash right now of about 16 gal of delo 400, 4 gal of Rot T GIII syn 5w40, 2.5 gal of M1 t &SUV, 2 gal of Mystik JT8, 4 gal of valv premium blue, and some Supertech 15w40 all to run in the psd. Can you tell which one is my top choice? But I have no qualms about using any of the above at different times.