I have recently purchased a 2003 Mitsubishi Galant ES/LS with 150000 miles and wanted to know what kind of oil to put in, so I looked at the owners manual and it said recommended 5W-30. I went to the store bought fully synthetic 5W-30 and did the oil change. When I was done the engine started knocking and so I called up a mechanic I know very well and he said because of the high mileage, it needs a thicker oil. He said that the parts are more spread apart inside the engine and it needs thicker oil such as 10W-40. Went to the store again and bought Castrol 10W-40 and the knocking stopped!! Is this true? Why would my engine knock with 5W-30 and not with 10W-40? Also in the owners manual it says to put oil certified with ILSAC or API SJ/CD. Isn't API CD have to do with diesel engine oils? This has really confused me for some time can someone clarify this for me please......