derek h - just a comment about judging the condition of oil by viscosity checking. Within oils or fluids there are additives that are also included in the oil/fluid. For example an additive in engine oil is a detergent which serves the purpose of cleaning the oil of by product from engine blowby and rust inhibtors, etc. After about 3000 miles of use in automobiles these protective additives rapidly dissapate but the oils viscosity remains relativly unchanged. This is one reason many people change the engine oil every 3K miles instead of the 6K miles recommended in the owners manual. People like me I speak of. 'Cause oil is always cheaper than metal.
Yes, I have performed these tests(viscosity & additive levels) in a lab in a 15 week long, fuels & lubricants course I once had. Not certain how you test the viscosity of oil in the field as viscosity is temperature sensitve & adjustments need to be made based on the ambient temperatures. When conducting viscosity tests and the equipment to test is a bit extensive and must be kept very clean.