I work at wal-mart in the automotive dept. The weights of oils all depend on what year car you guys use it in. the 10w means that in the cold it acts like a 10 weight oil. the 30 means in the heat it acts like a 30 weight oil. In a newer car (94 up) you only want to use 5w30 as the oil passages in a newer car and bearing passages are smaller and when you start that engine with a heavier oil like 10w or higher it wont get into those bearings as quickly on your start up. On a rebuilt I would deffinitly use a 5w for the protection. On an older engine thats used I would use a 10w for that start up protection. The only reason you would want to use like a 10w40 or a 20w50 is to slow engine leaks. And that harms your motor more than anything. It may slow the leaks but it harms the engine dramatically. During the winter you deffinitly want the lower weights. 15w40 is for diesels because its in their make of the motor to burn oil. hint hint, heavier oil is for burning oil engines. as far as brands go castrol gtx is the best. pennzoil the worst, and I hate to say it you guys but motorcraft oil is made by pennzoil/quakerstate. just read the back of the bottle. Also motorcraft filters are made by the honeywell corporation. which guess what. Also makes fram filters. I will agree they are made differently though. But they are still made by fram. Im sure i"ll catch alot of flak for saying that. Any questions or comments I'd love to hear em.