It is understood that the shearing of the polymers decreases the VIs.

However, I have yet to come across used oil analysis reports that considers a limit (caution/critical) to the degree of decrease in Viscosity Index over time.

Please advice on the recommended limit to VIs (both critical and caution level).

Thanks. John.

kumartr@enoc.com
When polymers are added to oil, the oil would not behave as newtonian fluid hence viscosity-temperure relationship is no more linear. Your question about fixing limits on VI would not arise due to the bove reasons. The best way to monitor polymer shearing (if it is multigrade engine oil) is to measure shear stabilty (30 cycle test) in this test KV is measured at the begining and after 30 cycle. KV is to be within SAE viscosity range. Mostly this test is done for fresh oil; it can also be done for used oils because polymer presence in the oil should not shear enough so that oil KV should not fall below SAE range. Before doing the, Please make sure that oil KV fall was not due to fuel dilution

Thanks Kumar.

A related query: with the addition of polymer into lubricant oil, any multigrade lubricant oil will behave as non-newtonian fluid. Why then, is VI used as one of the physical properties? Are these VI values only serves as indicators to oil's response to temperature variation?

John.

kumartr@enoc.com
Viscosity modifier (VMs)are added to alter the oil viscosty to increase at higher temperature and decrease at lower temperature (Pillow effect). If VM is not added oil viscosity will not either decrease or increase. Therefore Viscosity Index is not the creteria in judging oil condition in multigrade engine oil. There are many SAE papers to explain this mechanism.

Thank you for your comments, Kumar.

John.

quote:

However, I have yet to come across used oil analysis reports that considers a limit (caution/critical) to the degree of decrease in Viscosity Index over time.

I think most labs would have a condeming limit of when the oil "drops" off grade as in a 30wt drops to a 20wt then time to change out.
bruce CLS