Originally Posted By: friendly_jacek
Lets just clarify that the start up wear (a real phenomenon easily demonstrated in a lab) is related not to lack of lubrication, but to the cold engine problem.
Two questions. First, what's the basis for this? Second, what about the problem of the engine, even if briefly, turning over with little or no oil pressure? Yes, there are the residual EP and/or AW components that should still be on the metal, but this is still going to be a very challenging interval until pressurized oil arrives on the scene, so to speak. Another thing to consider: the Toyota hybrids, which do a lot of of/off with the gas engine actually use the smaller of the two motor-generators to spin the ICE up to about 1k rpms before the ECU adds fuel and spark. In effect, the engine pre-oils itself before it takes on a load. Why bother with this mechanism if start-up wear is not related, at least in part, to lack of lube? Beyond that, why would there be any market for pre-oilers, which of course, are meant to deliver oil pressure to the bearings before you start your engine? Beyond that, why would there be any need for assembly lube compounds for new or reassembled engines?
Originally Posted By: friendly_jacek
Water condenses on cold cylinders and combines with combustion byproducts forming carbonic and sulfuric acids. That produces chemical wear and not a mechanic wear. This is why synthetic oils may fall short in corrosion protection as being more viscous in room temp, they will run down and expose metals. Maybe this is why Mobile 1 produces more iron on UOA?
MAYBE, if an only if you start the engine, and then shut it right down. If you run the engine, sure, it takes the whole engine a bit to rise to operating temps, but I can't see the very hot combustion gasses allowing anything, other than perhaps the oil film, to remain on the cylinder walls for more than a couple seconds. I'm with Pablo on this one...
Originally Posted By: friendly_jacek
Then, lets answer the original question. The optimal wear is achieved with heavier rather than thin oil. The heavier oil will separate moving parts better than thin oil under pressure conditions. The reason thin oils are utilized successfully, is that ZDDP and other additives prevent excessive wear under full metal contact seen in thin oil under pressure (valve train). Thin oils provide better fuel efficiency as they decrease friction at the ring/cylinder interface at the expense of minor increase of friction in valve train and a potentially higher wear under heavy load and high oil temp.
The use of vague, subjective terminology ("thick" or "thin") reveals the flaw in this. You simply can NOT generalize in this way using such vague terms.
The most effective lube, in terms of viscosity, is what works best in any given engine.
If you drive a BMW M series car, then 10w-40 or 20w-50 is a dangerously "thin" oil. If you drive a Honda Accord or Toyota Camry, then 30, 40, and 50 wt oils are thick beyond optimal. And your reasoning about "thin" oils being used successfully because they contain EP/AW additives also does not fly. By this reasoning, you would need no such additives in your "thick" oils. But of course, they are most surely there.
You also can not generalize that "thicker" oils (whatever those are) do a "better" job separating moving parts than thin oils under "pressure conditions." Oil, like all liquids, is a non-compressible fluid. If a car is designed for, and in proper condition so that it keeps the right amount of oil, in the right place (bearings), and at the right pressure, a "thin" oil will keep parts separated as well as a "thick" one. Obviously, it gets much more complicated than this in the real world. Many factors all come together to determine whether a particular oil will work in a given application. Viscosity is just one of those.
Originally Posted By: friendly_jacek
Notice that 18 wheelers use 15W40 rather than 0W20.
I also notice that apples look, smell, taste, and feel different from oranges. It's simply a different lubricating context. They are operated differently, geared differently, frequently under heavy loads, and run in a totally different rpm range. Are you recommending that I use 15w-40 in my Camry, which specs 0w-20 as the primary vis?
Here's a good place to repeat my favorite broken-record question: if these "thin" oils (OK, let's just say 20 wt for now) are comparatively poor compared to thick oils then, almost a decade into their broad-based use, where are all the damaged engines? And if you want to make the general claim of inferiority for thin oils, here-and-there anecdotes don't cut it -- there would have to be a general pattern -- and there just isn't.
Originally Posted By: friendly_jacek
The example of racing is poor as as racing is all about max power at the expense of wear. Racing engines are torn down after each race and rebuilt.
Ah, now here's something that we are in full agreement about.