My 4.0 engine has nearly 200k on it with synthetic oil. Under the oil cap I can see some varnish, although I'm not very worried about it. Why is this a bad thing? Doesn't seem like it would make much of a difference in the operation of the engine to me...
a litlle varnish is the norm and in non moving areas like the top of the head the only thing I could see it causing is a little slower heat transfer to the oil, as some of the engine cooling is done through the circulation of oil but varnish/sludge there is a good indication of how much build up there is in other areas , varnish in the oil galleys it reduces flow. a little is no big deal a lot of slugde can cause major flow problems, build up on rings can cause them to wear and prevent oil from flowing in and doing its job od lubricating and sealing, extreme junk in the valve train can cause sticking also
if you have a little light varnish I would not worry about it
A coat of varnish (what we see a lot of here in the too-****ed-hot Southwest) not only is deleterious to valve stem seals, but to all seals over time as they are not receiving lubrication.
The varnish coat is responsible for internal engine parts not being able to be cooled effectively by circulating oil. A thin film doesn't seem like much, but it can be measured off of the valve stems and springs. They tend to get MIGHTY hot in normal operation (thus the varnish coat).
Auto-Rx sure is great for this. Oil cools not only the bottom end, but is responsible for the valvetrain components not overheating.
Granted, failure may never happen, but why let a potential problem exist that is so easy to fix?