[Razorback1980]

One thing I forgot to mention in all that is synthetic vs nonsynthetic. Synthetics are a little different. The rules I follow are this.

If I am using a nonsynthetic, I never put a 5W or 10W anything in my car. Here is the reason why. To get a 5W or 10W oil, you have to start with a lesser quality oil and add polymers to thicken it. Since polymers do not lubricate, you want an oil with less polymers. I also use oil with the smallest difference between the first number and the second. To get the larger gaps, they have to add more polymers and I want a minimum of those because not only do they not lubricate, but they also break down and leave sludge in your engine. I do use 20W50 in cars and I know what you are thinking 20W50 has the same gap as 10W40 (which I don't use). The difference is that 20W50 starts out with a 20 weight oil instead of a 10 weight. If I need a thinner oil for cold weather in a nonsynthetic oil, I use straight 20 or 30 weight. With my new cars which call for 5wsomething, I use synthetic oil because they don't start with a poor quality of lubricant to engineer that oil.

So...long story short here is what I do in Texas....I run standard 40 weight in the winter and 20W50 in the summer. If I lived further north, I would run 20 or 30 weight in the winter and 20W50 in the summer. If I wanted to run synthetic, then I would do 10W40 in winter and 20W50 in the summer.

The three oils that are exceptions (that I know of but there are probably more) to the rules are Mobil1, Castrol GTX, and AMSOil, they get multiviscosity oil without the polymers which is why Kevin hasn't had any problems running Mobil 1 and why so many high performance car companies recommend Mobil 1.