Fellas, fellas this is turning into a battle of the semantics. If only we had a oil flow meters fitted to our pickup tubes, we could put all of this to rest eh?
It seems we all agree that lower viscosity = more internal leakage which is effectively equivalent to oil bypass (without implicating the bypass valve - meaning whether bypassed or leaked internally, the resulting inefficiency is the same. This is regardless of the other factors of thin oil, like greater delivery orifice flow, the propensity for thin oil to flow more easily out of the bypass orifice at any given position, which is in turn determined by the backpressure acting on it.
With more viscous oil, internal pump leakage is lesser, meaning more volumetric efficiency, but efficiency is not the problem, so excess volume is bypassed anyway. To continue regulating pressure on the unstoppable positive displacement pump, the bypass valve is implicated (it's always implicated, but simply moreso in this example). More viscous oil requires the bypass to open MORE (the dynamic bypass orifice to be larger) in order to bypass the SAME volume of oil vs thinner oil. This requires a greater backpressure acting on the bypass to achieve the required increase in bypass orifice size to evacuate the same volume of high viscosity oil vs the low viscosity oil example. This is exactly why people see higher oil pressures with thicker oil, regardless of the bypass spring tension not changing and being 'set' to one certain pressure which in reality is dynamic and deviates from real world operating oil pressure.
tl;dr Pressure is regulated by diverting volume. Volume flow reduces with resistance to flow through fixed orifices on output side, and must be compensated for by a dynamic orifice (bypass), but the rate of flow through the dynamic orifice changes with oil viscosity. Therefore, the thicker oil will take more energy from the rotatating assembly to increase the 'head pressure' just to move the bypass into a large enough orifice size to bypass the same, required volume of oil. The increased kinetic energy required to non-productively move the increased bypass volume is converted to heat via molecular friction (which itself is greater than lower viscosity oil, beyond the already increase in bypassed volume) which counterproductively generates more heat.
Logically WRT pump 'efficiency', it's a wash with internal leakage vs volumetric efficiency. Addressing other variables would probably be more productive. The increased 'internal leakage' CATERHAM may have been suggesting by the use of thicker oil (and higher backpressures resulting) simply happens at the bypass valve, not so much in the trochoid's pressure section. The same effective result is happening, just via different routes and due to different variables.
It seems we all agree that lower viscosity = more internal leakage which is effectively equivalent to oil bypass (without implicating the bypass valve - meaning whether bypassed or leaked internally, the resulting inefficiency is the same. This is regardless of the other factors of thin oil, like greater delivery orifice flow, the propensity for thin oil to flow more easily out of the bypass orifice at any given position, which is in turn determined by the backpressure acting on it.
With more viscous oil, internal pump leakage is lesser, meaning more volumetric efficiency, but efficiency is not the problem, so excess volume is bypassed anyway. To continue regulating pressure on the unstoppable positive displacement pump, the bypass valve is implicated (it's always implicated, but simply moreso in this example). More viscous oil requires the bypass to open MORE (the dynamic bypass orifice to be larger) in order to bypass the SAME volume of oil vs thinner oil. This requires a greater backpressure acting on the bypass to achieve the required increase in bypass orifice size to evacuate the same volume of high viscosity oil vs the low viscosity oil example. This is exactly why people see higher oil pressures with thicker oil, regardless of the bypass spring tension not changing and being 'set' to one certain pressure which in reality is dynamic and deviates from real world operating oil pressure.
tl;dr Pressure is regulated by diverting volume. Volume flow reduces with resistance to flow through fixed orifices on output side, and must be compensated for by a dynamic orifice (bypass), but the rate of flow through the dynamic orifice changes with oil viscosity. Therefore, the thicker oil will take more energy from the rotatating assembly to increase the 'head pressure' just to move the bypass into a large enough orifice size to bypass the same, required volume of oil. The increased kinetic energy required to non-productively move the increased bypass volume is converted to heat via molecular friction (which itself is greater than lower viscosity oil, beyond the already increase in bypassed volume) which counterproductively generates more heat.
Logically WRT pump 'efficiency', it's a wash with internal leakage vs volumetric efficiency. Addressing other variables would probably be more productive. The increased 'internal leakage' CATERHAM may have been suggesting by the use of thicker oil (and higher backpressures resulting) simply happens at the bypass valve, not so much in the trochoid's pressure section. The same effective result is happening, just via different routes and due to different variables.
