I was reading a letter in Road & Track this month which was asking about oil viscosity and not using the manufaturers recommended oil. Well the only drawback that was mentioned was that it could probably affect gas milage if anything. Going on this, my question to you is what viscosity oil do you use and what is you average MPG? I am interested in how this might affect our cars. Oh, year of the vehicle would also be nice.

For me:
Oil: 10W-30
MPG: ~23
Year: 1995

10w-30 is fine for synthetic or dino. I personally do not like dino 5w-30 in the hot/humid southeast. 0,5,10 w-30 are all within oem specs.

The mpg between these xw-30 oils would be minimal. 10w-40 would be out of spec and get less mpg.

It depends on temperatures and length of driving time too. You will see a huge difference in colder weather with 10W-30 and shorter trips when you compare the identical car to 5W-30. The colder the weather and the shorter the trip, the bigger the difference.