For Nextcar's original question - that his car had sat for a few months on the dealer lot and thus he was worried about "time interval" aspect of the oil change...
First, understand why does oil need to be changed based on miles OR time? And how do car-like engines used in other devices (e.g. boats) specify oil changes since they don't have odometers? What's the rational used - how did we end up with "7,500 miles or 12 months, whichever comes first" type of things for road cars?
Most non-car piston engines specify oil change intervals based on number of HOURS the engine has been run. Boats and aircraft typically have hour meters on them. Those motors often spend a far greater percentage of their operating time at or near high power too... so "hours" gives a pretty good indication of revolutions the engine has made and thus how many times the oil has circulated. That boat or airplane engine runs at high power routinely... unlike a road car engine that is asked to generate numbers like 12HP to 20HP (steady highway cruise) for most of its life, 100HP runs for typical accelerations from a stop, and the occasional lead-foot blast at full power. But "full power" is barely a few percent of the engine's running on a car, unlike a boat or aircraft. So "hours" is a much less reliable way to determine "usage" of a car engine. Ergo we get miles... which also saves the car makers a few bucks since they don't have to add an "hours" display - they just use the existing odometer. The "usage" based oil change interval (hours or miles) basically is intended to cover oil breakdown from physical abuse inside the engine and particulate accumulation. Particulates come from air+fuel or combustion gasses that manage to sneak past (blow-by) the piston rings (no rings seal 100% perfectly) and that don't get sucked out by the Positive Crankcase Ventilation System (PCV valve & vacuum hoses). Those particulates are actually a little acidic... they accumulate in the oil and slowly eat the engine from inside once the oil absorbs all it can chemically block/handle. Also, some teeny metal bits continually wear from engine parts too... gotta flush those out once in a while. The oil filter captures some of this of course, but not all.
The "...or 12 months" part of car engine oil change intervals comes from humidity. The air brought into the engine for the combustion product of course has whatever the outside/ambient humidity level is. Remember the blow-by? On a stone-cold engine, any humidity in the blow-by gasses may condense into water along the insides of the cold engine block or cold oil pan. Water in the oil is A VERY BAD THING for the rest of the engine. Water not only is damaging to the engine bearings, it also reacts with the blow-by particulates to form those acids. So changing the oil every few months or every year is necessary to get that water out.
Rather than trying to fine-tune/tailor the miles/time intervals to different areas of the country (after all, Phoenix Arizona has less humidity than Seattle Washington, right?) and having a nightmare for warranty claims, and/or making it a nightmare for owners to keep track of, manufacturers basically set the limits based on the "worst expected" conditions. So folks in Arizona probably could get by with more than 12 months but the "simple" owner's manual guidelines don't say that.
As for "normal use" versus "severe duty" - what is that really? It varies from manufacturer to manufacturer but a big chunk of it is how often is the vehicle driven only a few minutes before being shut off again? Folks that do a lot of short drives - so the engine barely warms up - stress the engine and oil a LOT more. First, a stone-cold engine block and cold pistons have some physical dimension/size, right? But as the engine warms up, thermal expansion makes everything grow a little bit. The pistons have to grow quickly - after all, they're front-n-center with the heat source! So they have to be "undersized" when stone cold so they have room to grow before the block warms up and starts growing too. That means the piston ring to block clearances are largest on a cold engine... so that's when the engine gets the most blow-by particulates/contamination. And the colder the engine, the more likely any trapped humidity will condense into water. More blow-by = more trapped water so it's a double-whammy.
If you get the engine & oil hot enough for a while, the water will evaporate out again... and get sucked up by the PCV system. Not running the car long enough for this to happens hastens how quickly water accumulates in the oil. So that's one part of "severe duty" - lots of short trips that don't let the engine run a full temps for a while. Idling is also hard on some engines; in carb days and early fuel injection days there had to be "excess" fuel in the air+fuel mix just to make sure enough gas got into the cylinders to burn. Big cylinders (to make big horsepower) fed only a teeny bit of air (all that is needed to make idle horsepower) and little fuel end up with the air+fuel molecules spread out in the cylinder - they may not be close enough to spread the burn reliably. A little extra gas makes sure the flame spreads. But any unburned gas slides down the cylinder walls, past the piston rings, into the oil pan. Unburned gas is about as bad as water in the oil; it prevents the oil from bonding to metal parts (e.g. the engine bearings) so those parts get denied oil protection. This problem is nowhere near as bad today with modern port fuel injection but it's still not totally gone either. So lots of stop-n-go driving, with lots of idling, puts gas into the oil too. Ergo change it more often to keep the oil clean ==> severe duty.
From a car engine's point of view, steady highway driving is the "easiest" of all. The engine has a chance to reach total thermal equilibrium so there are no parts still expanding while others are not - differential expansion is a source of physical stress in the engine. The air+fuel mixture is optimized for best emissions so 99.999999% of the gas burns and goes out the tailpipe; very little should remain "unburned" to sneak past the rings and end up in the oil. And the engine is plenty warm enough to evaporate any trapped water or gas so the oil has a chance to clean up a bit. And highway miles are low RPMs yet the miles accrue rapidly - far more rapidly than slower speed driving in town with normal stop lights, idling, spending time in 1st, 2nd, 3rd, etc. gear, and whatnot. So "1000 highway miles" might be only half or a third of "1000 city" miles worth of engine revolutions ==> half to a third as much physical wear.
Since I live in SoCal, with generally low humidity levels, I pretty much ignore the "...or 3 months" factory specified oil change intervals on my older cars. They get it based on miles or 1 year, whichever comes first. They don't get lots of short trips either. They're long out of warranty so it doesn't matter. For my still-under-warranty Genesis though I do follow the miles and time factory intervals - I don't want to give Hyundai any reason to deny a warranty claim.
mike c.
