Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
Slick and most similar date from the 1960's when 20W/50 multigrade mineral oil was a state-of-the-art product. They may have helped some then. I built a tuned and modified engine in the early 1970's and it was then regarded as good practice to coat cams, followers and newly rebored cylinders etc in a 50/50 mix of Slick and oil because it tended to stay in place rather than run off components whilst the engine was being built, so the motor didn't start life dry. Whether it did any good I don't know, but it didn't do any harm.
However modern synthetics and component tolerances are so far in advance of what we had then, that good oils are now astonishingly good. What's not so good is that those improvements - the fact that synthetics just don't break down at temperatures that murdered mineral oil in 3-6,000 mls - have been seized as a marketing opportunity to enable extended change periods. The oil is still good after 12 or 18k as a lubricant, but - as ever - accumulates a burden of carbon, metals, acids and water that eventually make it a source of wear, corrosion, gum and varnish. No additive can fix that, at least not without compromising other qualities.
The sole class of pour-in additives that can still be useful now is detergents, that can dissolve and remove gum and varnish deposits back into the oil. Which is what flushing additives do, with the expectation that they're used briefly immediately before an oil change to get rid of the crap. Most specify 10 or 20mins max, idling only, because the detergents badly degrade the oil as a lubricant. Which is why, although all motor oils contain detergents, they only contain a small amount. Flush additives work effectively, for that one specific purpose, but even then they're a bit controversial because in theory they can loosen solidified deposits that could cause problems by blocking oilways etc. I've never heard of that actually happening, but it could. And there is a benefit of removing gum and varnish, especially if the engine has had neglected oil changes in the past, which itself can restrict oilways, can cause sticky operation of components like valve gear.
As everyone else has said: use good synthetic oil, change it at sensible periods, 3,5 or 6,000 mls depending on how fastidious you want to be, and most engines now will last as long as the vehicle, barring poor design or materials, component failures and owner abuse that aren't oil-related.