HI Murat,
Viscosity is the most important parameter for an oil, so the specification data sheet must be correct and representative. However, oils supplier can sale the oil with 5% + or - the specification. If the precision is very important, so you must do this test in a laboratory, why not in two laboratories (ASTM D445).
For the second problem (foam), i suggeste to read "Using Oil Analysis To Monitor the Depletion of Defoamant additives) in noria_learning_center, or other topics in this same center, a lot of trouble shooting are given.
oil Analysis: What you need is: spectrometric test (element analysis : wear element, additives and contaminants), viscosity, TAN/TBN (acidity or alkalinity of the oil) PC (particle count for filtrated oil), and water (by KF coulometric analysis).
Note that a can found a huge information on oil analysis in noria.com.
Djaouida
I suggeste