What Type Oil
 

What type oil would everyone recommend I use in my 68 coupe, 302 4V? During the restoration process over the past 3 1/2 years (which it hasn't been driven, just cranked every weekend), I've changed it every 3 months with 10W30. A mechanic a couple of days ago said I needed to use 10W40 in the winter and 20W50 during the summer. Any other thoughts?

You don't state where you live, but I would think 10-40 would be a little stiff in the winter, unless you have an oil burner. I use 10-30 year 'round.

I live in southern Mississippi. Average highs/lows in the summer around 92/70, and in the winter around 59/45 (although we have a very short winter). Not sure if the mechanic is suggesting heavier oil because the engines been sooped up with all high performance parts or not. 20W50 did sound kind of thick to me though.

I found this on a website:
Multi viscosity oils work like this: Polymers are added to a light base (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

Multi viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter base your decision on the lowest temperature you will encounter, in the summer, the highest temperature you expect

Look on the back of the oil bottle to determine the viscosity for your temperature ranges.

Synthetic oil
 

If you use synthetic oil like Mobil 1, then you can use even lower viscosity such as 5W30 because synthetics don't thin even as much as regular multigrade oil. This ability to run lower vicosity reduces the windage in the crank case and will gain you a few horse power. More expesive though.

Rev