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Maybe just to sum-up, any oil will varnish if given the right conditions and enough time - conventional or synthetic. Having said that, synthetic oil resists varnish and sludge formation much longer than dino under a given set of operating conditions. So to answer your question, most synthetics will do just fine with a 5K OCI (and longer depending on the engine itself). For many engines and driving styles, there are those here that claim that 5K miles on dino is fine. It wouldn't be in my engine, but it certainly is in say a Honda engine or the GM 3.8L V6. Again, depends on the engine, how it's driven (mostly highway or mostly stop and go/short trips), adn the conditions (do you live in Phoenix or northern Michigan). All of that plays a role.