If varnish is being formed then the lubricant has been hit with a high temp spike,
In a well designed system we would expect any good quality lubricant to work so figuring outwhere the high temp surface is could find why,
Unless this was a new lube in the system and formed varnish I would be reluctant to "blame the lube,
Pop some more detail up and we should be able to sort thru and figure why this is occuring?
Rob S