Hi, all. I have a 2004 Pontiac GTO that I'm currently using to commute to work. My daily commute is 120 miles round trip, 110 miles or so is steady, 70mph highway. The manufacturer recommends Mobil 1 5w30 synthetic, however... I've tried many others including Royal Purple, Pennzoil Platinum, and Mobil 1 0w30. I was spending a good bit of money on oil changes (which I did roughly 6k-8k miles), so I decided to give something new a try: Valvoline Maxlife synthetic-blend for high mileage motors (I'm just shy of 75k on the odo). Will there be any ill effect from making this change? I plan on sending a sample to Blackstone once I drain this batch. I also switched from K&N oil filters to WIX. Half the price I was paying before! :) This is the first day since the oil change, and the engine's former valvetrain racket seems to have become muffled. I assume this is because the 0w30 was a bit thinner, but the valvetrain used to be very loud even when the car was at operating temps. She just sounds healthier now. I'm wondering if this is all in my head though. Anyways, I just wanted to see if there is anything wrong with switching to a semi-synthetic or if it will offer less protection than the others. I did a little bit of research before joining, and mixing seemed to be ok. Thanks for reading my mini-novel.