Yesterday I drove about 100 miles, much on the freeway, and the car felt sluggish. I was getting concerned that it might need some work ... plugs, pcv valve, or some such. But this morning I realized that the last oil change was with M1 EP HM 5W-30, the thickest oil that's been in the car since I've owned it. In the past, the oil's been M1 EP mixed 0W-20 & 5W-30, providing a substantially lower viscosity. Can't help but think that the heavier oil may be contributing to the sluggishness and poor mileage.
I'm tempted to drain out the HM and replenish with some 0W-20 EP and regular 5W-30 EP, both of which I have on hand. It's a waste of good oil, but I hate the way the car is performing, and I'd like to see if it's the heavier oil that's contributing to or causing the poor performance.