dielectric change

I want to sink my rewound transformer in oil. Is it unwise to use oil 
that has been exposed to air, humidity, dust etc for extended periods of 
time? Will the dielectric constant change to the point of making the oil 
useless as an insulator? If so are any oils immune to contamination ? ( 
no fancy oils, just common and easy to find )
Thanks much ,
Ron Q.  
Cambridge Ontario                                                   

Get Your Private, Free Email at http://www.hotmail-dot-com