It's mostly noticed when you have a older car and the manufacture of the car recommends a lower velocity oil than it is said in the owner's manual. Witch can be death. Noted as the Mazda RX-8 oil issues as you would get a oil change at the dealership the oil put in would be a 5W-20. In the car but the Mazda RX8 oil manual recommends using SAE 5W-30 engine oil. At idle the lighter oil was not lubricating the engine enough and all around anyway and the oil should be higher anyway as it's found out!
"The reason we recommend this slightly thicker rating (over the Mazda recommended 5w30), is to help protect you from stationary gear bearing failure. It is important to use a semi-synthetic oil rather than fully (synthetic) because semi-synthetics will burn reasonably cleanly and leave fewer carbon deposits behind in your engine; reducing the risk of apex seal obstruction which can lead to serious problems, resolvable only via an engine rebuild." http://rotaryrevs.com/garage/help/best-oil-for-rx8
its best to put in what the manufacture of the car says to put in it. like the oil in America says to use a 0W-20 but in Japan they use a 5W-30 and they are the people that made the car but also know Japan doesn't get colder than 40 to 50 something degrees so a 5W-30 would be good in Japan but in colder climates a 0w-20 is what the manufacturer said to put in it so.
The point is to know about all of this mess about oil. It's your car it gets you to your destination on time so do what's best for the car!
