Out of curiosity, I dumped the ML 5w20 early and refilled with ML 10w30 (latest formulation), which has a NOACK of 10% and a VI of only 140. (drained 5w20 will be used at a later date in the Mazda) My reason for doing this is 1. To see how oil consumption is vs the 5w20. 2. See how the oil holds up with fuel dilution, which the 2.4L's are known for. All of my UOA's for this engine, and UOA's I've seen on other 2.4L's, come back showing very low Flash Points. I want to see how a low VI oil (140 in this case) handles fuel dilution and oil consumption. Viscosity modifiers are often a problem in situations where fuil dilution is an issue. There are exceptions to this such as M1 0w30/AFE and Amsoil/RL etc. Those oils attain their VI mostly through very high quality base oils, and use little VII's. This past weekend I put 300 miles on the 10w30, and much to my surprise, the car actually felt better than it did on the 5w20. Far from scientific with no data, but the power at high end was more noticeable and the engine responsiveness is great. I averaged 80-90mph for sustained periods of time. If I had to guess, I'd say maybe the piston ring seal is a bit better with the 10w30, creating a tad more power. I will be curious to see how the UOA looks in about 5,000 miles. After running the MaxLife 10w30, I'm going to run their Full Synthetic MaxLife 10w30, which has a NOACK of 6.3% for comparison. One of the SynMax PDF's suggested a 10w30 is best for a high mileage vehicle. Being I tend to drive fast on the highway, and WOT now and then, I think a 10w30 is a good choice. If you call Redline, they often recommend 10w30 and prefer oils with no VM'ers for high performance applications. The Honda 2.4 is far from a high performance engine, but my highspeed highway drivng may benefit a bit from an oil with no VII's.
http://www.synmaxperformancelubricants.com/PDFs/SynMax_Tech_Street_MotorOil_General_Information.pdf *Mobil 1 AFE 0w20/30 provided the lowest oil consumption to date in this engine, which goes against the idea that oils with no VII's are in some cases better. I attribute this to the very high PAO content on both AFE oils.
Q: Should I use lighter or heavier SAE weight oil in my older OEM application? A: You should look in the original OEM manual and follow best as possible the recommended SAE specification. In genenral, over 100,000 miles use 10w30 instead of a 5w30.