Originally posted by eskimo2
So how often do you think oil should be changes?
eskimo
Actually, its a moving target. There are several variables I can think of:
1) Average trip duration
2) Ambient temperature conditions
3) Ratio of cruise-to-stop&go operation
4) Age/state of wear of engine
The period of engine warmup is by far the greatest problem area in terms of both engine wear and pollution of the crancase oil. Internal tolerances and oil viscosity are optimized for the engine at normal operating temperature, therefore you must reach that temperature before your lubrication system functions as well as it should. Plus, fuel mixtures and burn efficiency are likewise best when the engine is at normal temperature, your fuel burn is much more dirty both in terms of emissions and pollution of your crankcase while warming up.
So, the more a vehicle is used for short trips, the more stressed your oil will be. In addition to the factors I mentioned already, whenever you shut off your engine and allow it to cool, there is a tendency for atmospheric humidity to produce condensation inside your crankcase. The small amounts of moisture introduced to the internal environment of your engine's lubricated parts combine with by-products of the fuel combustion in the crankcase to create acidic liquid pollution of your oil, which filters simply cannot strain out (they are only good for particulate contamination).
Obviously, colder climates aggravate this.
The nature of your trips is also a factor. An engine in a cruise state at normal temperature is most efficient in terms of fuel mileage, and also in how clean its running. Vehicles running at steady cruise state pollute the air AND their oil much less than vehicles that are accelerated and decelerated frequently.
As Roscoroo correctly pointed out, engines that are older and have more wear pollute their own oil more than newer "tighter" engines.
All this makes it difficult to set a precise mileage for optimal oil change intervals.
Fleets with the best management nowadays utilize oil sampling programs that tell them what's best for their fleets based on laboratory analysis. They leave the oil in there as long as possible, by establishing through sampling just how long it meets acceptable standards.
Obviously, its pretty much impossible to get that precise with individual vehicle owners, they vary in their matchup to all the factors too much and laboratory analysis of oil samples isn't as economically feasible.
Its true that some folks, for instance folks who have engines in really good shape, who tend to start their engine, warm it up quickly, and use it for long intervals at mostly cruise speeds, may be OK with 10,000 mile oil changes. However, that's relatively few people. The more you use shorter trips to accumulate the same number of miles, the colder your environment is, the more you stop and go while on a trip, and the older your engine is, the more frequent your oil change interval should be.
My experience taught me 10-15K miles per year is average for the typical motorist, and that this mileage is achieved via 2 to 3 trips per day. In that duty cycle, I'd recommend a seasonal oil change (Spring-Summer-Fall-Winter).
Hence, the traditional rubrick of about 3K miles per oil change....which is IMO a good rule of thumb for most people.
culero
