This topic comes up every now and again on the Rennlist 993 Forum. He's a review by a knowledgeable person regarding this issue and the differences between OW-40 vs. 15W-50. Personally I use OW-40 in my ML and 15W-50 in my Porsche and my 300E.

"0W-40 is probably the perfect all around choice for 98% of street driven 993s. I think it does have an Achilles heel that make it not so good for some owners, of whom there is an especially high incidence on the Rennlist forums.

To get a 0W-40, even starting with a synthetic base stock, you must begin with a rather thin oil, probably what would be the equivalent of a 0W-20. Then you add viscosity extenders to it, which are long chain organic molecules that unfold as they meet higher temperatures. This keeps the oil from thinning as fast as it would normally as it heats up, and allows it to have the 40 weight rating at 100 degrees Celsius.

One problem with viscosity extenders is that they aren't lubricants themselves, so their volume displaces the lubricating base stock, and the lubricity of the resulting oil is actually a little less. The bigger problem is that under high temperature, high shear conditions (high rpm!) these molecules are sheared apart and destroyed - over time. As these molecules are destroyed, the oil becomes thinner at high temperatures, so what was originally a 0W-40 oil starts approaching the viscosity of the original base stock - the 0W-20 stuff. This is not good for continued use at high temperatures and loads.

This is probably unlikely to happen to 0W-40 to a significant extent under normal street use within a recommended oil change interval. But if you drive your car hard on the streets of Phoenix in the summer, or regularly take your 993 to the track for DE events, 0W-40 might not hold up very well. This is why track guys like synthetic 15W-50, which has a higher viscosity base stock, and uses little (maybe none) added viscosity extender and will provide proper protection to the engine under "race" type conditions for a longer time than 0W-40.

So you see why Mobile 1 0W-40 might be the perfect oil for GJ, but not at all the right one for Greg's race car. There are no simple answers!"