friends truck 2011 1500 Chevy with the 4.8, he bought brand new and it currently has 61k miles. I started changing the oil for him at 40k, at 5k intervals. Besides one oil change where i used 5W30 PU and the current fill of 10w30 M1 EP, i have always used dexos spec'd oil in it from 40k to the current 61k. Cheapest filter i used was a Promotive once, usually use an ac delco pf48 or a fram ultra. Even before i started changing it, and still to this day, i usually crawl under it after to check for leaks and such, i always hear a slight tapping noise coming from the bottom end, it also does it when the engine is completely warm and has had a good run on the road. so pretty much all the time. Its not very loud, but noticeable. Could it be the infamous piston slap these vortecs are known for, or was the problem fixed by '11? With the short 5k runs on synthetic, im thinking of just using Castrol GTX synblend 5W30 from here on out, which is dexos specd, with an ac delco filter and calling it a day. good choice? Thinking of adding half a quart of either lubegard biotech or MoS2 to each oil change or is that not necessary?