The type, weight, and brand you use depends on what you intend to do with the car.
Synthetic oils have two major advantages: Heat resistance and long drain intervals. Most cars will easily go 12,000 miles between
oil changes. And
synthetic oils will handle 250*+ for hours at a time without a break down.
A true
synthetic oil starts with a PAO base stock, not dead dinos.
AFAIK, there are only 4 PAO oils on the market: Royal Purple, Redline,
Amsoil, and Schaffer.
All other oils sold as a synthetic are really not. Due to a legal trick of marketing, they can sell a dino
oil as a synthetic if it has been processed enough to meet certain performance requirements. Doesn't make it a bad oil. But it does make it a deceptive marketing plan (IMO) and not necessarily a good value.
Oil gets hot from extended rpm's, like road course or open road racing. For street driving, drag racing, or autocross, Dino oil won't get hot enough to break down.
Dino oil work just fine for the vast percentage of vehicles on the road. I use Royal Purple in my air cooled Harley and racing Cobra. UOA supports this. Everything else gets dino oil.
Mobile1 would not hold up for 4,500 miles in my air cooled Harley.
ZDDP was reduced in gas and diesel motor oils because it can clog up the cats. Doesn't hurt the O2 sensors, though. If you don't have cats, I suggest a decent additive. I use Comp Cams break in additive.
Viscosity is another big point of confusion. And here's where you really have to pick carefully. Viscosity of a petroleum product is most affected by temperature. It's usually measured at 40*C, and that gives you the basic viscosity of the product. For example, a 40 wt oil will have a viscosity of 40 at 40*C. But it will have a different viscosity at different temps.
The first number (ie: 5W) tells you that it will have a viscosity of 5 when it's cold, usually measured at 0*C. The second number (ie:40) tells you that it will pour like a 40wt that's been heated to 100*C. At that temperature, it will NOT have a viscosity of 40.
For the first number, you want it to be very low, so that the engine will start easier and the oil will pump quickly to the bearings when cold. The colder the oil is, the thinner the oil should be. Most engines should do well with 0wt.
The second number is the operating viscosity. You need enough viscosity to maintain good oil pressure. More than that puts a strain on engine components and causes drag. That makes the engine less efficient, builds more heat, costs you power and mileage. Look at the recommendation for a lot of new cars - 0W-20.
20W-50 is NOT the correct oil for the vast majority of engines.
Some race engine builder say that a 40wt is needed for valve train stability in racing conditions. But most say that a 30wt is perfect. Some NASCAR teams are using a straight 0wt oil.
As far as oil changes go, that again depends on the vehicle, use, and type of oil. Let the UOA be your guide. My diesel pick up gets an oil change every 15,000 miles. The air cooled Harley every 8,000.
If you use a dino oil, change it every spring. Over the cold winter months parrafins and waxes can congeal and form sludge.