The question of engine oil viscosity and oil pressure is a complex one and
perhaps the discussion to date is a little over simplified, since a critical
issue is how a journal bearing (i.e. crank shaft and rod bottom end) works.
A reasonable explanation is presented at
http://www.machinerylubrication.com/article_detail.asp?articleid=779
although it isn't really written for automobile engines, but the principles
apply. Here we find that too low a lubricant viscosity may allow for metal
to metal contact in the bearing and too high a viscosity can create
cavitation damage on the bearing surfaces. The kicker is when things get
worn and clearances increase sufficiently, a higher viscosity may be
required to lubricate the bearings which may also result in restoring the
oil pressure to a more normal level. The internal pressure in the bearing
that keeps the shaft and journal separated is generated by the rotation of
the shaft, not by the pressure from the oil pump. This internal bearing
pressure can be up to 100 times the oil pump pressure. The oil pressure
supplied by the pump is that pressure required to supply the bearings with
sufficient oil flow to supply the bearings needs (to balance what leaves the
ends of the bearings) and to supply clean, cool lubricant to the bearings.
So, as any good consultant would say, the answer is, "it depends," and
getting back to Gary's comments, he's right not to focus exclusively on the
pressure issue because we are trying to make a bearing work properly (as
well as supply other lubrication needs like the valve train, pistons, etc.)
and the pressure gauge is only an indication of the lubrication system's
function, not an exact measure of how well the system is functioning.
The above discussion merely helps frame some of the issues involved. I am
not an automotive engineer, but practical data must exist in the engineering
departments of the auto makers to provide design specs (oil viscosity, pump
volume rate and pressure) for engine lubrication systems. If any such data
actually existed for the TR6 engine, it probably is long gone. It may have
been experience-based anyway, so perhaps the best we can do today is to
continue to trade our experience about what worked for us.
BTW, I use Castrol 20W50 in my 6 and it stays nice and clean because it
leaks out before it loads up with dirt/water/combustion products/etc.
Bob
73 TR6 BRG
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