Is there a standard or dielectric constant number that when reached lets you know an oil change is needed?

from my experience, anytime your dielectric changes by .02% something is going on with your oils chemistry.

Years ago, was a company which had a small meter which supposedly would compare original sample to used sample and let you know if oil was still usuable.
My experience with it was that only fleets could keep samples of original oil--and most wouldn't. In non-fleet use, reformulations were often enough, especially with AMSOIL, that readings were inconclusive.

Dick - I believe you speaking of the portable OilCheck - it is still used by the US Military and municipalities to extend drain intervals:
http://www.reliabilitydirect.com/oilanalysisproducts/SKF-TMEH1.htm
To answer cccous, there is not a standard number, you are looking for a change from new sample to the used sample.

No standard value as dielectric constant varies greatly between base oils and additive packages. Then it typically changes a lot with temperature. If dielectric constant has changed by probably a few percent (at same temperature) this could indicate something significant.

There is evidence that measuring the dieletric loss of the oil, which nearly always rises in use, gives a more useful yardstick for the oil condition.

Relationship of dielectric constant of an oil with the quantity of soot, insolubles, moisture, metal particles and unburnt fuel is a very interesting topic and needs some more discussion.Once this relationship is established, a small instrument would be able to tell when to change oil in a diesel engine.
I have a doubt, some contaminants increase the dielctric constant and some others decrease it,they may cancel each other.Generally both are present in the oil and their net result may not reveal the true condition of that oil. Then how some equipments based on change of dielectric contant are being still used by US military for oil-changing decision? Will somone clarify my doubt ,please.