Re: Oil types
Any truly concerned with use of 5W-xx lubricant in older vehicles should check the recommendation that FoMoCo made in the owners manual.
I repeat that I don't think 5W-xx or 0W-xx should be used in older vehicles as low pressure at lower rpms could be the result. The main and rod bearing clearances were a bit looser when those engines were manufactured. Check the manual.
If you want to use 5W-30 instead of 5W-20 in warmer climates/months, the only thing that would happen is that your mpg might be reduced by .03%(or similar), or maybe it was 3%, but I don't think it was that much.
As a note, oil pressure indicates that the pump is supplying more than the galleries and bearing clearances can leak out. If you had perfect oiling, pressure would not be needed, except to overcome friction of flow, and the 'work' it takes to displace the oil already in the bearing clearance.(between journal and bearing surface) With higher viscosity, same temperature, the oil will be a little thicker and thus a bit more reluctant to flow. With larger clearances, the 10W lubricants will start with a bit higher viscosity than would an 0W or 5W based oil. If the lube is too thin, it will flow more readily out from between the journal and bearing, and perhaps not develop a good 'wave' inside. The parts of the bearing, journal and insert, are supposed to never touch in a plain bearing engine. The lubricant keeps them apart except at startup, with the parts riding on a 'wave' of lubricant. FWIW.
tom
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