Yes, an electrostatic oil cleaner is excellent for pre-filtering new oil to an NAS Class 6. As a matter of fact we have alot of experience at filtering new oil which is generally a NAS Class 9 or 10 right out of the drum.
If you are able to "kidney loop" filter, where you first pump all the oil out of a drum into a clean tank and then hook the electrostatic oil cleaner up to the tank so it can circulate filter the entire product before then filtering it into the reservoir you will reach your NAS Class 6 much quicker and easier.
I would suggest using an Electrostatic oil cleaner that is both fast and efficient. Check out
www.oilkleen.com for more information.
Also, there seems to be some questions on this posting about varnish removal. Check out the link below to see before and after photos of real varnish stripping from internal metal surfaces. This is the true test of any technology claiming to remove varnish. If your vendor can not produce before and after pictures like this then they really are not removing varnish.
http://www.youtube.com/watch?v=b_FkV_SWETU