Speaking of HDEOS in particular (although the same evidence can be seen in PCMOs), if you peruse lots of UOAs, you'll see that low mileage UOAs don't show any favorable wear reduction comparing dinos to syns. At least, not anywhere equal to the cost ratio.
The syns gain their advantage when the OCI goes up.
In short to moderate OCIs, dinos can hold their own right along with syns, regarding wear protection.
I understand that there are claims of other synthetic benefits, but most nearly all the time, those benefits are never actually achieved by the user. Let's review some of them.
1) fuel economy: might be a bit of savings here; I'll admit that. But it's very slight; not the 10% you often hear touted. Fuel economy is most controlled by driver attitude and style; the foot is the main contributor/detractor to efficiency! A realistic view of efficiency gain is likely 2% or so, if you stay in the same grade. Castrol claims a 5% gain when using their PAO Elixion, but that is comparing a PAO 5w-30 HDEO to a dino 15w-40 HDEO, so not only is there a base stock upgrade, but also a grade viscosity reduction! If you compare syn to dino in the same grade, I suspect 1 or 2 percent would be "normal". If you get 15mpg with dino, you'd only get a few tenths better with syn! You want better economy? Get off the loud pedal!
2) better cold temperature range; certainly true. But to what gain? Dino oils are available in many grades. Pick one that works for your environment. HDEO syns would really only have a significant advantage over dinos when the temps fall well below zero, say -15 deg F or lower? How many of us have that happen on such a routine basis that we NEED synthetic? I certainly agree that SOME small percentage of our members do, but certainly not the vast majority. People want the great low temp advantage of syns, but they don't NEED it. A 10w-30 dino HDEO will function well below zero. Heck, my Dmax owner's manual recommends 15w-40 dino down to zero!
3) better hot temp range: Again, true. But in such a small window that it's a moot point. What do I mean by that? Consider this ... If you get your engine hot, just how hot is hot? 210 Deg F is fairly normal for the coolant temp in today's turbo diesel trucks. Certainly the EGT pushes the temps in the turbo as high as 1500 deg in very extreme circumstances. But, the oil is never in there long enough to cook. (It's like waving your hand over a candle versus holding your hand over the candle). Oil is not sitting stagnat in the turbo; it's moving. The heat soaks into the oil, but not the FULL temp, because the heat transference is being assisted by the flow. If this were not true, the coolant in your engine would not function either! Cooling is a function of volume and velocity. So, dino oils can handle the hot turbo just fine, because they and the engine were both designed with this performance need in mind. The only time a synthetic would have a true advantage is if you actually overheated your engine, via water pump failure, etc. And the synthetic would actually withstand the temp rise better than dino, BUT only for a short time, and then it also would become overwhelmed just as the dino oil does. So, the "hot" protection offered by syns is really actually a very narrow windown of "too hot" for dino, but "not yet over heating". That's a pretty narrow window, so your chance of actually being in that exact range for the benefit is pretty darn narrow as well.
4) syns are cleaner: really? I disagree. Here's why. Let's agree that it's very likely that the rate of contamination is fairly steady. The soot generated by the combustion process is fairly constant, over the OCI period. Dino oils are designed to handle soot and contamination just like any oil, because they have both detergents and dispersents. The dispersents are actually more important in my opinion. They help keep the soot and other insolubles from agglomerating (co-joining) and become physically larger. When soot is held small (typically it starts out sub-micron) it cannot do much harm; most data shows that most engine damage occurs in the 5-15um range. So as long as your oil is keeping the insolubles below that size, little damage is done. So as long as the dino oil does not have it's add-pack overwhelmed, it can function just as well as the syn. You see, the contamination rate is the same, so even if the syn has "more good cleaning stuff" in it, it does not make any differece until it would surpass a point where the dino oil is saturated, but the syn has some additive left. Syns don't clean better; they clean longer. It's the very same philosophy as wear protection; syns don't protect "better", they protect longer.
