[56markII]

Since I don't live in Arizona or Furnace Creek AND if I'm lucky I have a summer that reaches 90 degrees instead of 50's or 60's for temps up in the Snow Mexican Republic of Canuckastan�� and for over 30 years all I ever ran in my 50 Mercury with a 255 is 5W30!
Around 80,000 miles on it since I rebuilt it in the 80's and I still get 40-50psi at idle and 75+psi on the highway and yes my gauge is working properly. For those who think they need oil as thick as gear oil look under the hood of a modern Ford truck and notice 5w20 on the oil cap! For years Oldsmobile has had 5W30 recommended including in the big boats with 455Cu. Another reason for light oil other than keeping the engine cleaner by better splash is the valves are only lubricated by mist which is cast off the connecting rods and flies up through small holes in the lifter valley. I ran 15W40 one year and when I pulled my car out of winter storage many of the valves were sticking until I let the engine roar on fast idle for a while. It's comical listening to valves snapping shut after the engine is shut off and is cooling down. Oh yes not only do I use lighter oil, I even tossed the oil deflectors snapped on the oil lubrication pipe in the valley to allow more oil to fly up there with NO noticeable increase in oil consumption after all the valve stems are hanging down and not pointing up like an overhead valve engine! A little experiment I did so I leave them off because even if consumption increases a tiny bit, it doesn't bother me.