Re; General qtn. lube viscosity - pls. advise

Open gear systems generally move slowly and are applied as total loss systems, hence the requirement for high viscosity oil and good adhesion.
Basic principles for lube oil viscosity are high load,low speed needs thick oil and high speed, low load then thin oil.
Hence high speed bearings use oil ~ ISO 5 and heavily loaded slow speed gears ISO 1000-1500.
All other applications in between with hydraulics ~ISO 15 to ISO 68 depending on factors such as ambient and working temperature.
Thicker is not better i.e. in the case of high speed bearings,even slightly too thick oil will cause seizure in some bearings due largely to tempearture rise, affecting clearances.

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but a better EP oil with higher film sbut a better EP oil with higher film strenth of the same viscosity may do a much better job.
trenth of the same viscosity may do a much better job.

Agree 100%