Originally Posted By: CharlieBauer
I found this as interesting as the study itself:
Secondary Research
Oil companies have evaluated the performance of conventional and synthetic engine oils by sampling
used engine oils from real-world vehicles. AAA reviewed published research in this area to understand
real-world performance. In one study, a matrix of almost 500 used oil samples were obtained from
vehicles operating in and around Houston, Texas. One hundred and twenty of the nearly 500
samples were formulated using full synthetic base stocks. Within the samples, numerous brands and
additive chemistries were represented.
A rapid decrease in oxidation resistance occurred in mineral-based engine oils after 3,000 to 4,000 miles
of use. In some cases, very low oxidation resistance was found after 2,000 miles of use. Significantly
better results were observed for synthetic based lubricants. The authors did not attempt to
attribute this to the better stability of the synthetic base stock or the specific additive package. It was
also found that synthetic-based engine oils retain the ability to neutralize acids appreciably longer than
mineral-based engine oils. It was concluded that many of the better performing engine oils subjected to
real-world use were formulated using synthetic base stocks
One needs to exercise a bit of caution in interpreting this kind of thing as often as not, the collectors of the original data aren't impartial bystanders and have their own agenda to push (like encouraging you to buy more profitable synthetic over less profitable conventional).
Yes, it's highly likely that after 4k, a conventional oil will 'have less resistance to oxidation than a synthetic' but that's not the same as saying it has NO remaining resistance to oxidation. The same goes for TBN.
Oxidation per se isn't actually a problem until things reach a tipping point, at which manifestly bad things occur. This usually happens all at once when the oil's viscosity goes through the roof, highly condensed/polymerised gunk drops out of solution as sludge and so much acidic stuff is floating around oil that bearing metal starts disappearing at a rapid rate of knots. At a very rough guess, I'd say most conventional US oils wouldn't reach this truly bad situation until you go beyond 12k miles. And yes, synthetics will very likely not reach this point until much later (I took one of my own synthetics out until 15.5k and it was fine) but the reality is that almost no-one in the US extends their OCI this far. So where is the true benefit??
If I put it another way, at 61 years old, I have 'less resistance to death' than I once did. However that doesn't mean I'm in imminent danger of falling off my perch tomorrow and another 10 - 20 years might be quite doable!