The formulated high mileage oils are better to use in high mileage engines. They have different seal conditioners than you see in standard oils and are formulated with different or more viscosity index improvers, dispersants, detergents, friction modifiers along with several other things. Additives are a waste of money because, generally speaking, a lot of science goes into the additive packages that are in oil, and most of the stuff you buy over the counter are just snake oil made to make you feel better. Everyone knows if you put more money in a hole, you will think that everything around that hole works better.
The over the counter additives are not tailored to each specific oil and its additive package that you put in the car, and therefore have to be made fairly generic. This means that some of the additives may react poorly with the oil you are using, or may end up doing more harm than good. Too many friction modifiers, too many detergents, sulfur compounds, etc. Oils you buy over the counter are not a one size fits all formulation, and there is a hell of a lot more in your oil than just oil, so over the counter additives just really can't be made to work well in oil. The detergents in an additive may effect the antiwear additives in your oil. The friction modifiers in additives can lead to early breakdown of the hydrocarbon chains that make up your oil because of the sulfurs in them.
Just stick to a high mileage 5w30 with an API service designation of at least SJ, and you are golden. Honestly though, if you are not burning oil and your engine is running like it always has, just stick to a standard oil formulation. Don't waste your money on additives, and don't listen to anyone who talks about their racing back in the 70s or is telling you from their years of experience owning cars. Like I said in a previous post. Go to BITOG and look at what some real tribologists and chemists say about oil. Engine oil technology has changed a lot in the last few decades, particularly the last decade. No one should be basing ANYTHING they do to an engine on stuff they did before the 90s.
Also, just to add to things. Paraffin base oils do not cause sludge. That is a myth and a stupid one. EVEN IF there was enough wax in the oils that there would be some type of clogging problem, ask yourself a question. What temperature does paraffin wax melt at? The wax is removed in the refining process and sold as other products. Also, Kendall oil is not, and was never 100% napthenic base oil. Paraffinic oil is actually BETTER at resisting sludge forming tendancies because it, as it is well known, it is better at resisting thickening and thinning at high or low temperatures and it has much better oxidization resistance.
You should ONLY, and I repeat ONLY change the viscosity of oil you are using to a higher viscosity if your engine is not getting enough oil pressure at operating temperatures. You should ALWAYS use the lowest viscosity oil that gives you proper oil pressure at operating temperatures because that means better start-up lubrication since the oil can flow faster and get between parts better. Start up is where most of the wear in an engine comes from, and that is what you should concern yourself with the most.
