These engines were designed to use straight-weight oil, usually 30 weight in the summer, and 20W in wintertime. BTW, "W" stands for "Winter" not "Weight" as most people assume. A 5W-30 oil, for instance, has the flow characteristics of a 5 weight oil measured at 32 degrees F., and the lubricating properties of a 30 weight at full operating temperature, usually around 200 degrees F. It DOES NOT change thickness, remaining at the thickness of a 5 weight all the time. That's too thin for the clearances of a stovebolt engine, even a relatively late model one like a 261. I'd reccomend a 15W-40 diesel-rated oil for year round use. It had the extreme pressure additives including zinc, that are necessary to make the flat-tappet cam live a long and useful life, and the 15 weight base oil is thick enough to maintain good oil pressure. With good bearing clearances, I'd expect to see about 40 PSI or slightly more at a cold start, dropping to 25-30 PSI at hot idle. Don't get upset at low pressures at hot idle, 20 PSI or less, as long as the pressure rises somewhat when engine speed increases. Most "idiot light" warning systems turn on at 7 PSI or less. I've seen engines live a long time at 0 PSI idle pressure, and 15-20 PSI at cruise RPM.
Jerry
