V6-2.7L,Oil consumption. Burning oil. Oil light turns up on idle.

Since I bought my car I noticed that it consumes a lot of oil. The last owner used 10/30w semi synthetic oil. When I have the oil changed the seller told me to use 15/40w oil to strip the oil consumption. I get an oil change about every 6 months. Recently when I drive the car for 15 minutes, when I stop at a red light at idle, the oil light turns up, and immediately turns off when I press the gas pedal. This happens when I'm in D drive, R drive. I checked the PCV valve, it was so sludged, so I cleaned it with carbon cleaner. Nothing happened and the problem was still the same. I haven'