Re: Service Query
the oil pressure relief valve maintains a constant oil pressure just after the pump, but the viscosity of the oil is critical to maintaining that pressure in the oilways, remember pressure loss over any given distance is directly proportional to viscosity. the oilways have the correct length and width to suit the recommended viscosity of oil, thinner oil should have thinner oilways to maintain the same pressure over the same distance. this means it is impossible to have the same pressure at the variator with a lower viscosity oil.
without monitoring the difference 5w30 would make it is only a matter of opinion if the pressure drop at the variator will be significant. I know people have suffered premature camshaft lobe wear (especially on cylinder 4 which is at the end of the oil supply) on HCS ford engines when they mistakenly use 5w30 because they think is suitable for HCS engines (when it is only suitable for zetecs), but that does not mean other engines designed for 10w40 will also suffer similar oil circulation problems on 5w30, however I would want to take the risk.