Entire books have been written on these subjects, but I'll attempt to provide some insights into the general
subject for the enlightenment of non-engineers. A lot of issues regarding lubricants boil down to common
sense, combined with some technical understanding of what lubricants do and how they work.
In another thread, some asked for an expansion on my response, so I'm putting it here for greater visibility
and for the benefit of all.
Everyone knows that lubricants lubricate. That's why they're called lubricants, right? Well, there's a bit
more to it than just that.
In an engine, lubricants help metal surfaces slide past each other without contact, or if in contact, then
without doing a lot of physical damage due to scraping and grinding effects. They also do another
important thing: they cool the pistons. Water/anti-freeze/engine coolant cools the engine block and
cylinder liner (or cylinder in a solid-block engine). However, you can't get efficient heat transfer from the
piston which is also heated by the flames in the combustion stroke as each engine goes through its two
or four strokes per combustion cycle. Consequently, oil is pumped from the galley in the engine into the
crankshaft, then up through the connecting rod to the piston pin are where the oil is sprayed out onto
the piston head and skirt where it can pick up heat from the piston itself before dripping back into the
sump (oil pan).
Part of the oil that is sprayed onto the piston seeps out through the oil ring or elsewhere and onto the
cylinder wall where it is scraped off by the oil ring around the piston skirt or head, depending on where it
is positioned. A thin film remains on the cylinder wall and acts as a lubricant between the compression
rings (above the oil ring) and the cylinder wall. As the piston moves downward, a small amount of oil
remains on the cylinder wall, and is burned off when exposed to the heat of the burning fuel on the
cumbustion down-stroke. To reduce wear on the rings and cylinder, metallic or other additives are put in
the oil, and when the oil burns away at high combustion temperature, a minute amount of metal (or
additive chemical) is fused to the cylinder wall. These metals are usually copper, perhaps zinc, or other
metallic elements, and they provide a non-steel wear surface to keep the rings from scraping, galling, or
damaging the cylinder wall in other ways. Thus these additives add considerably to engine life due to
dramatically reduced wear.
As the connecting-rod bearings rotate around the crankshaft, oil from the crankshaft core (obtained from
the galley at the main bearings then conducted to the connecting rod "throw" where it goes to the
connecting rod and up to the piston) is also used to lubricate the connecting rod bearing. The inside
diameter of the bearing, held captive by the rod and half-circle cap, is very slightly larger than the outside
diameter of the polished bearing surface of the crankshaft throw. This leaves room for the oil to act as
a mechanical separator, keeping the bearing from being able to directly touch the crankshaft metal at
any time, as soon as the engine starts cranking for start-up and continuing until it is shut down.
The oil escapes into the space between crank and rod bearing whenever the position of the holes in the
shaft throws is located such that there is space between crank and rod (bottom of the rod during combustion, compression, or exhaust in a 4-stroke-cycle engine, for example). As the crank rotates, the
oil attaches itself to the "boundary layer" of oil film adjacent to the crank-throw surface, and the
inside-diameter surface of the rod bearing, surfaces which are, in turn, attached to the metal (very
loosely, of course). The drag placed on the oil by the layer-to-layer adhesion of the oil to itself
(governed by the viscosity -- the higher the viscosity the higher the adhesion) enables the engine to
"pull" the oil from the side where the distance between bearing and crank is greatest around to the
side where they are closest. The thinner the oil film gets between the two surfaces, the more force
is available to drag the oil into that narrower space, thus "pumping" the oil into that area so it can
develop enough pressure resisting the mechanical force coming from the engine's operation that it
keeps the metal surfaces separated. This is the principle behind the "journal bearing" used on early
train axles (before roller bearings that were widely adopted following World War II), sleeve bearings,
and such. Even "oil-less" oilite and sintered bearings that have oil impregnated into the bearing
material use this principle, drawing oil from the bearing by the heat caused by rotational friction.
The force required to pump oil into the pressure side of a journal (crankshaft) bearing increases as the
oil viscosity (stiffness) increases. If the journal tolerances are too tight for a given viscosity (not enough
space between bearing and shaft), the oil cannot distribute properly because it takes too much pressure to get oil through the crank passages and into the journal and the bearing fails. If the viscosity
is too low (too thin), the force available from the oil to keep bearing and crank separated, the metals
touch and the bearing is destroyed.
Engines operating at high speed (RPMs) with very close bearing tolerances require thinner oils in order to
keep oil in the lubricant space between journal bearing and shaft. Thus, 5W20 and 5W30 oils are often
specified for these engines. Diesels -- especially big diesels -- on the other hand, run much slower and
have wider spacing between bearing and shaft. They also have higher forces and pressures between
shaft and connecting rod, and therefore require thicker, heavier oil, such as 20W40 or straight 40W.
Most automobile gasoline engines for several decades were specified to use 10W30 oil, rather than 20W in winter and 30W in summer, or some other combination. This was because most consumers are too
ignorant (uneducated), too lazy, or too stupid/dumb (doesn't matter which of these is the case -- you
still get the same result) to know how to manage different viscosities, and retailers have hammered
suppliers to simplify their inventories of motor oils. For engines that specify 10W30, I always run 30W in
all seasons except cold winter months (December to mid-February) and 10W30 during cold months. I
get phenomenal engine life with no problems, and Valvoline, my preferred oil, keeps the engine interior
clean and sludge free, unlike Pennzoil and Quaker State, which I avoid like The Plague. Walmart stocks
30W for use in lawn mowers, but their shelves overflow with 10W-30, reflecting the simplification of
Engine motor oils perform many other functions, such as collecting acids and other nasty products of
combustion and neutralizing them or keeping them in suspension so they can't harm the engine. They
react chemically with contaminants, and since the oil chemistry depletes with time, it is important to
change the oil at specific intervals (3-4000 miles on cars, longer for buses and trucks running diesel
engines). Otherwise, nasty stuff attacks engine components and you get bigger service bills.
Having oil analyzed for chemical contamination and metal wear particles can tell you much about the
condition of the engine and whether the oil is doing its job. For best results, stay close to the engine
or vehicle manufacturer's recommendation for viscosity, quality or grade, and oil-change intervals. An
engine is an expensive piece of hardware and lasts longest when properly maintained and cared for.
Crankshaft main bearings support the crankshaft in the engine block structure. These bearings work in
essentially the same manner as crank-throw, piston connecting rod bearings. The only difference is that
they do not send oil to any other engine location, but receive oil only. Excess oil squeezes out the sides
of the bearing-to-shaft gap and drips back into the sump, as do the connecting rod journals (except for
the oil that goes up to cool and lube the pistions).
The purpose of lubricants in transmission is to cool, lubricate sliding or mating (gear) surfaces,
and provide a mechanical means of connecting power source to load through the torque converter in
an automatic transmisison. For manual, geared transmissions, 90W or multi-viscosity 90W120, etc.
are typically used. The lubricant is "pumped" between the gear teeth by means of the same principle
that is used in journal bearings. If one gear tooth's face directly contacts the face of another without
the oil/grease barrier, you get galling and other damage, severely reducing gear life. If the gear teeth
never touch, such gears can run essentially forever. Differential and ring-and-pinion gears also work by
the same principles.
Now we come to tapered roller bearings that are ubiquitous in axle hubs, differential gear carriers
(pumpkins), and similar situations. When the bearings are inside the rear axle containing the differential
and ring-and-pinion gears, the usual 90W or similar gear lube is nearly universally used. It is typically deep
enough in the center of the housing that the oil goes out to the ends of the axles where it also
lubricates the hub/axle-end bearings.
These tapered roller bearings consist of cylindrical rollers held apart by a roller "cage", which is a conical
ring with rectangular holes to match the bearing rollers. The ring and rollers are placed between the
inner and outer "races". The races have a cylindrical outside diameter (outer race) and cylindrical inside
diameter (inner race). Each race has a conical (tapered) surface with a lip on each side so that the
bearing rollers can roll freely between the two races, but cannot escape the fence-like ridges on each
side that prevent the bearing from coming apart. In operation, tapered roller bearings are used in pairs.
The cones in a pair of bearings are oriented so that as the outer race is captured and held in an outer
carrier or housing, and the inner races are attached or held to the rotating shaft, and pressure is applied
to both bearings by means of collars, nuts, or other restraining devices against the inner races such that
they are pushed toward each other and the pressure causes the two tapers to reduce the clearance
between bearing rollers and the inner and outer races of each bearing is forced to zero and pressure is
placed against the rollers so that they actually apply significant force from roller to each race. This allows
free rotation with zero slop or movement. High-grade tapered bearings are also commonly used in metal
lathes so that the spindle runs extremely true with no run-out or wobble.
A tapered roller bearing theoretically can run without lubrication for a very long time, provided there is no
argument between the rollers and the cage that holds them in place. However, this is the real world.
To ensure cool, smooth operation, heavy axle grease is packed into the area between races and
between rollers so that there can be no friction-caused heat build-up and so that the edges of the cage
openings don't drag on the bearing rollers, leading to damage and bearing failure. At least that's the way
most hubs have been done for many years when the bearing is not part of a drive axle with lots of oil or
differential grease/lube drenching the hub bearings on the axle housing.
However, heavy grease is not necessary, nor is there anything "sacred" about 90W oil. All that is
necessary is a lubricant that keeps any contact between the cage and the rollers cool and smooth so no
damage can occur. Thus, some oil caps for steering-axle hubs are filled with 30W or 90W oil. One could
just as well use 40W if that is what is being used in the engine. When a manufacturer specifies an oil for
such situations, using the recommended oil keeps the warranty in place. Using an "unapproved" oil might
possibly leave the manufacturer an escape hatch should a failure result in a lawsuit. Often times the oil
recommended or specified was used in a test sequence so the manufacturer knows it is adequate. In
such cases they may prefer to avoid suggesting other lubricants because they were not tested, and
therefore the life expectancy or reliability is unknown, and they don't want to accept any liability should
the result be satisfactory.
So, as usual, some of this stuff boils down to money.