Well, I have just read 4 pages of opinions and conjecture about the lubricating properties of engine oil versus engine wear characteristics. I'm going to postulate that there are two primary factors concerning 'excessive' engine wear, the operating temperature while operating and the amount of sulphuric acid in the oil when the engine is off and cold. After I get your blood boiling over those topics, I will pontificate about several other visual indicators that mainly turn off the 'average' automobile affecionado, such as varnish and sludge, which sometimes is also referred to as 'dirt'.

TEMPERATURE
Oil is used to both lubricate and cool the moving parts of an engine, keeping friction, which causes wear, to a minimum.

Oil temperatures lower than the minimum viscosity value, even under no-load, low rpm conditions, can cause wear to increase by a factor of 8 to 10 times depending on the viscosity of the oil at the low extreme (the '10W' portion of the 10W-30 viscosity specification). Excessive wear on bearing surfaces will continue until the oil is heated to flow properly and provide the necessary oil film, or friction barrier. Do not confuse the oil temperature with the coolant temperature. An automobile engine's oil does not reach operating temperature until the vehicle has travelled an average of eight miles. The coolant temperature is reached within a mile or two because the cooling system thermostat stops the flow of coolant to the radiator until the coolant reaches the minimum temperature. Oil temperature thermostats are used only with aircraft piston engines, for obvious reasons (if it quits, you can't get out and walk).

Oil temperatures higher than the maximum viscosity value (the '30' portion of the 10W-30 viscosity specification) causes the oil's lubricating properties to break down which in turn allows friction surfaces to heat up past the temperature limits of the metal itself, causing severe failures such as piston seizure and bearing failure that may ruin the entire engine. In this case, the high temperature thins the oil excessively, decreasing the friction barrier and allowing the bearing surfaces to come in contact with each other.

Temperature factors seldom cause engine wear to the extent that enthusiasts cringe with embarrassment because of low-mileage overhauls. Unless a given enthusiast lives near a highway and cannot wait for the engine to warm up sufficiently before he blasts down the road at ninety miles per hour, he will normally be spared the humiliation.

SULPHURIC ACID, THE UN-ADDITIVE
Any respectable auto maintenance manual, like Chilton's, Motor Manual or that other one that I should remember the name of, will contain a section with photos dealing with the deadly substance (to piston engines, anyway), sulphuric acid. If you have not read about sulphuric acid, do so because you cannot consider yourself a REAL enthusiast without knowing what the number-one engine killer is. That would be akin to calling yourself a Christian, but having no knowledge of Satan.

Sulphuric acid is created in your engine oil pan (sump, if you insist on euro-speak) without your doing any more than driving the less-than eight miles per day to and from work each day. I will not repeat the factors that cause it's formation--look it up and spare me the effort of copying it into this thread. Take it from me--the previous thread's definition is factual unless you want to split a few hairs. Sulphuric acid mostly affects the metallic compound commonly known as 'babbit', that is, the very soft material used for engine bearings--main bearings, rod bearings, cam bushings, etc. These bearings, after a prolonged attack of sulphuric acid, look like they were decorated by an insane etcher. Instead of flat, smooth surfaces, the bearings look like an etch-a-sketch surface created by children. Because the bearing contact area has been reduced by as much as 40-50%, the bearings wear out much faster, requiring an early major engine overhaul.

Sulphuric acid does not go away on it's own--it will not evaporate--if the oil is not changed, the damage continues to occur. For this reason alone, vehicle manufactorers recommend the oil change every umpty-ump miles OR three months. The three months is to ensure that any sulphuric acid is eliminated at least every three months.

Several others in this thread have ventured to say 'old folks' are responsible for the myths concerning the frequency of oil changes versus engine wear. I'll counter that assertion with my opinion that most enthusiasts, except the most diligent (age notwithstanding) do not know what the hell they are talking about. To prove my point, search this thread for the term 'sulphuric acid'. If I recall correctly, there were, at the very most, two occurrances (at least one, though). Then, to really prove that you want to learn about this particular subject, do an Internet search on the same term and any of the keywords from this thread that tickle your fancy, such as 'motor oil' (NOTE: I do not think 'dino oil' will get you any results--by the way, who coined that trite term? Dawn Rainforest? While you are at it, see if any oil companies are dumb enough to posit that their super-duper 100% extra-virgin synthetic oil will prevent the formation of sulphuric acid. If you spot any, expect them to follow up with an announcement that their new handy-dandy in-the-privacy-of-your-driveway invention will rejuvenate your piston rings and replace the babbit in your engine bearings by adding their brand of 'motor honey' to your oil and molybdenum pills to your gas tank. With their super additives, spark plugs that fire in oil, a propeller under your carburetor and an ionizer on your gas line, you will be ready to challenge the Challenger to a race to space.

THE INFAMOUS CHECKER CAB EXPERIMENT
People continue to misconstrue the facts concerning this experiment. The cabs did NOT travel less than eight miles, shut down, cool off, drive eight miles, etc. They ran, albeit mostly at idle, at least 18 hours per day BUT THEY DID NOT COOL OFF--HENCE THEY EXPERIENCED NO 'EXCESSIVE WEAR'. Neither do city busses, tractor-trailer rigs, police cars, UPS trucks or on-the-road salesmen's cars. This is not rocket-science here.

Another note on temperature and oil viscosity--Here in the Southwest U.S., the weather gets hot--really hot. Despite this fact, the manufacturers up in Detroit insist that 10W-30 oil is fine for summer-time driving, Possibly so up north, but down here when the asphalt road temperatures reach 130+ degrees and you are tooling along at 80+ miles per hour with your air conditioner blasting, the highway speeds will wreak havoc with your engine oil. Because the temperature rarely drops below freezing in winter and stays above 95 degrees in our six-month long summer season, I use 20W-50 oil exclusively. Especially when the temperature is less than 50 degrees, I try to loaf along if possible before getting on the highway, to let my engine oil at least get warm. If you pay attention to your engine, you can tell when it gets warm and is ready to go.

VARNISH AND SLUDGE
Varnish is mainly an indication of a neglectful owner. I think of it as hard sludge. I may be wrong, but the only harm I have seen from varnish (if you want to consider it harm) is that it builds up in the piston ring grooves, but in fact it may actually increase the tension on those tired piston rings.

Sludge is a yucky, gunky substance that can clog the oil return passages and/or PCV valve, but is mainly an ugly reminder of a dastardly neglectful previous owner. If so, you should have spotted this abuse before you purchased the car, because if the previous owner neglected something as important as engine oil changes, you can be sure other maintenance was not performed either. A quick and easy way to spot sludge on any car is to examine the underside of the oil filler cap and inside the oil fill hole, usually in the camshaft/rocker arm cover. Evidence of varnish and high heat can be revealed by examining the oil and transmission dip sticks above the level marks for varnish and discoloration.

If you find any traces of 'dirt' inside your engine, ask your significant other if they let your child make mud cakes on top of the engine with the hood up. Otherwise, I would suspect a disgruntled co-worker, neighbor or cuckholded husband. If none of these suspects are evident, the next time there is a flood in your area, move your car to higher ground.