Ah,..the good old "I change it every 3000 miles & never had a problem, so I must be right" myth. In PART ! of my reply, I explain where the 3000 mile figure came from.
Part ONE: "Mineral Oil? Filter? We don't NEED no stinking filter!"
Early internal combustion engines were lubricated with oil from castor beans, the infamous "castor oil" that used to be administered as a laxitive. This was really, REALLY hard on early pilots, as the exhausts and engine leaks exposed them to a steady diet of castor oil fumes - thus the "dashing young men in their flying machines" were actually dashing...to the latrine! Clearances and seals were so loose that the oil probably never stayed in the engine LONG enough to go bad, as it was contantly being replaced. Filters? Heck many of the engines of the day had NO OIL PUMP - the rods slamming into the cankcase splashed oil all over - called "splash oiling". Fast forward to the '30s - engines are starting to use "mineral oil", refined from petroleum, and with additives. In the interest of longevity, most now have at least a partially pressurized oil system - still no filter though - a simple screen over the pump intake is all thats there. Air filters are primitive, if present at all, and the metallurgy of bearings, cranks, valves, and blocks is only slightly improved. The fuel contains tetra-ethyl lead, as both a "top end" lubricant, and an octane enhancer. The question now arises, since the newer technology is sealed better, "How long do we run on one "change" of this new oil? Engineers and chemists got together, greased up their slip sticks, and decided for aircraft and heavy equipment, 50 hours of operation was about right. When it came to cars, slight problem - no detailed logs of operation are normally kept, nor is there a timer for hours of operation - so they "swagged" an average speed of 30 MPH, multiplied it by 50 hours, to get a recommended oil change interval of 1,500 miles.
FAST FORWARD to the '50s: The modern "spin on" oil filter is coming into use, much better than a screen (which is still there) at purifying the oil - everything has at least an "oil bath" air filter, if not an oiled paper-element or foam filter for the air. Automatic chokes are starting to reduce the time the engine runs excessively rich, which reduces oil contamination by unburned fuel over "manual chokes". Modern overhead-valve systems replace "flathead" valve-in-block technology and REQUIRE a fully pressurized oil system - as a side benefit, mians and rod bearing oiling is also improved. Real advances in engine component materials are starting to be seen. Vastly improved additives are now in "motor oil". NOW, how long do we run on one "oil change"? Again, airplanes led the way - 100 hours was thought to be safe. Again, they "swagged" 30 MPH,...and came up with the "3000 mile oil and filter change".
The '60's and 70's- blocks get tougher - the automakers, who want to avoid using expensive aluminum for blocks and heads, add tin and other alloys to cast thin, strong, "lightweight" iron castings - which also are more wear-resistant. Technology for seals starts to incorporate neoprene, nylon, and other synthetic materials - bearing metal improves...
The '80s up to now - What few carbs are still hanging on are computer controlled - everything else is one form or another of fuel injection - ELIMINATING fuel contamination of oil. Synthetic oils, and conventional oils with improved additive packages cope with foaming, coking, acid, dirt, viscousity break-down, and moisture MUCH better. Thermostats are 15 degrees hotter, lead has been removed from gasoline, removing another oil contaminant. Oil filter technology improves, with some "extended life" filters claiming lifetimes in excess of 15,000 miles, aircraft and major fleets use oil analysis to determing if oil needs to be changed - how long do you run it in your car? Especially when some manufacturers are recommending oil change intervals in excess of 7,500 miles? The answer in part two....