This is a generic question - but the subject vehicle is a 2005 GMC 4.3 engine. Sleeve-type bearings are lubricated because a hydraulic oil â€˜wedge' is formed as the shaft rotates relative to the bearing. This is also true of roller type bearings, now that I'm thinking about it, as each roller acts as a rotating shaft - just on a smaller scale.
But I am really asking about bearing design / performance in terms of a â€˜bushing type' of bearing - such as an auto engine or a large hermetic or open-drive compressor.
And in that context:
What benefit does oil pressure provide? More / higher oil pressure is not what actually supports the rotating shaft inside the bearing - it is the hydraulic-wedge which is created by the rotation. So: so long as sufficient oil is provided to ensure a full layer of oil on the bearing - isn't any higher oil pressure just a wasted effort?
I ask because I recently installed a good-used engine in a 2006 GMC truck. The old engine (essentially a small block chevy ) always ran the 5W30 oil that GM specified. And it always showed about 18-20 lbs oil pressure at hot idle. Maybe 40 lbs. cold at speed - with about 30 lbs. hot at speed.
New (er) engine initially idled hot at about 10-12 lbs and ran at about 20-25 lbs. on the road. That troubled me so I asked the shop what oil they had used. They claimed not to know, saying only: "whatever was the right oil for that engine."
I use Mobil 1 in everything, and I doubt they used that, so at about 1000 miles or so I changed the oil. Truck calls for 5W30 but it lives in south Florida so I thought: oh I'll just use 10W40 instead - I don't need any 5 weight oil characteristics.
Now the cold idle/speed pressures are about 35 & 65 - with the hot idle/speed pressures about 25 & 60 lbs. Which was sort of â€˜comforting' to me.
All of which has made me ponder the actual benefits of higher oil pressure. I now theorize that the engine shop used 0W20 as that is what many modern Asian vehicles use - and that was why my initial oil pressure was so â€˜low'. OK; so now I have oil pressure which seems 'normal' to my virtually antique mechanical mind - but . . . is it actually better for the engine? Thinking about how oil actually acts inside a bearing, and also about how all the newer engines use physically thin oil, am I making a mistake in judgment? Maybe I am just wasting horsepower creating a higher oil pressure while it is actually no actual benefit to the engine?
Yes; it does make me Feel better - but is it Actually better?
What do you all think?