Using modern synthetic oils rated for your vehicle , using a good oil filter , regularly checking PCV valve for proper operation and OCI's of less than 5K miles or 12 months max - this is all I can think of to reduce the likelihood of sludge & varnish ... Your thoughts and experiences ?
This topic is waaaaaayyyyy to complex and varies across all manner of criteria to have a one-size-fits-all answer.
- First of all, sludge and varnish are different. Sludge is accumulation of combustion byproducts and oxidation of the lube, obviously not good, and needs to be avoided. OTOH, varnish is harmless discoloration of surfaces; nothing to worry about.
- Next, the engine design lends itself to being either clean running, or having a proclivity to sludge. As discussed in another recent thread, there are things that make for good or bad conditions (see that thread).
- Then, the lube chosen is a factor; what kind of base stock? What cleaning agents? What capacity for holding contamiation in suspension? Etc
- Finally, the OCI duration, relative to the factors above.
There are engines that have exhibited a propensity to sludge up even using synthetic lubes and 5k miles OCIs.
There are engines that have shown to be very clean running even when pushed out to 15k miles with conventional lubes.
There are examples of lubes that have good wear traits, but leave much contamination behind.
There are examples of lubes that not only run clean, but scrub off years of previous contamination loading.
You want a specific answer, you have to ask a specific, detailed question covering all known parameters and products and conditions.
You raise a generic question, you deserve a "
Well ... it depends ..." answer.
The factors you mention may or may not be enough to ward off sludge.