So what I'm wondering about, now that my vehicle is getting older, is what weight oil I should be running during the warm weather months, like April-September. I have always been running 5w30 during the winter and 10w30 in the summer generally and have had no problems whatsoever. However, just about to turn 111,000 on my '01 Z24 and going to change the oil soon that I will run prolly till September. I've read how important it is to maintain the oil level on the LD9 engine and bearing failures are guaranteed if you let it run below 4 qts. Given my mileage and also importance of maintaining the oil pressure on this motor, was thing of going to 10w40. Any opinions? FYI I run Mobil 1 (was thinking about High Mileage Mobil 1 also)

During the summer I run straight 40w Mobile1. I'm in the Deep south where it stays hot. The 10w40 are okay but have chemicals in them to be thinner when its cold for easier starting. The main thing is keeping your oil changed regularly like it seems you are already doing.

yeah, we're real good about the oil changes. I've never run anything but 5w30 (winter) and 10w30 summer. Reason I asked is I read somewhere out there that oil pressure is real critical on the twin cam (quad 4) and an older engine is better off with the heavier stuff to help maintain minimal op specs. I actually thought that running the heavier stuff was detrimental because newer engines (maybe last 20-25 years) have much tighter tollerences than in the old days thus you want run thinner oils to maintain proper lube of the bearing surfaces, but I might be totally off base with that.