Originally Posted By: DBMaster
You have to admire the passion displayed on this site! Not only are most convinced they are right, they also have to make sure everyone else is wrong. Armed with a little knowledge we are all online degreed chemists and petroleum engineers.
Mobil only backed off its claims of 25,000 mile oil changes in the 70's due to automotive industry pressure. Was the oil they made in 1975 inferior to what they make today?
Since engines today use far less fuel to travel a given distance my limited logic capability tells me that there should be fewer contaminants in the oil. Furthermore, with fuels being formulated to produce fewer emissions that would suggest that there are fewer combustion byproducts to make their way into the oil.
I beg your pardon if I am suggesting I am a chemist. I'm not.
Let me put it this way (easier to understand):
(a) there's a huge design and implementation differences between the gasoline IC engines in the 70s and nowadays(ref: most standard EFI engines under OBD-II type management) is that (a) back in the days of yore, oil contamination especially fuel-related diluation (carburettor is a very archaic, crude way to get fuel mix with air, and a lot of unburned (still liquid form) fuel gets into the engine as a result). While I don't doubt anything GpIII or beyond (since the days of Mobil 1) have very robust base oil, unfortunately, oil still gets diluted with unburned fuel during the course of in-engine operation. Fuel diluation is somewhat cumulative in a sense that while frequent, extended oil-temperature operation will help evaporate some of them trapped ones somewhat, there's still quite a bit of them left behind, mixed with your engine oil and "thins" it out.
(b) moisture in all forms including exhaust gas from combustion chamber bypassing ring gaps, will still get into the oil and mixed it up. Gasoline mix in the days of yore contains generous amounts of sulphur, which mixed with moisture, formed acid.
(c) base oil degradation due to thermocycling, shearing, and other mechanical, chemical if not thermal means.
(d) most conventional motor oils back in the days of yore comprised of mainly solvent-scrubbed mineral-base oil (Gp 1 primarily) on API SE grading, which comes with a lot of impurities with poor base oil viscosity (must add a lot of Viscosity improvers in order to meet the standards for certain API mutli-viscosity grading).
(e) very archaic ignition and emissions systems design (points-based, no cat, in N.A. PCV valving already in-use since the 60s, etc.), all contribute to the accelerated contamination of the engine oil itself.
(f) small sump for typical passenger grade automotive gasoline engines.
With all these in mind, 3k OCI on general gasoline-based automobile makes sense.
While you may argue with me that the base oil is very, very robust these days (citing more boutique base oils, some runs beyond typical GpIII, etc.)
Do remember that we still have to combat the following even with the latest and greatest computer-managed EFI or DI engines. Some fundamental challenges still out there and have to be dealt with:
(a) engine oil fuel-dilution problem
(b) moisture creation during the combustion process
(c) base oil degradation due to thermocycling, shearing, and other mechanical and chemical interaction while engine oil in-service.
(d) PCV valving and also EGR (exhaust gas recirculation).
(e) relatively small oil sump in typical passenger grade automotive gasoline engines.
So long as there's no perfect energy conversion mechanism achievable on the surface of this world, gasoline IC engines still more or less have to face these challenges and engine oil still see these problems as a major concern.
As I said before:
Oil filters take care of insolubles. Changing oil filters still does not elminate the fuel dilution trapped in your base oil, and neither does the dissolved contaminants.
I can see that there's the possibility of extending the engine oil service life during the oil filter change (small amount of engine oil remains inside the old oil filter gets changed out while fresh make-up oil added into the sump.
Lastly: highly-refined base oil (Gp III or higher) may be robust throughout the OCI service interval, there's no good way to replenish the otherwise depleted additivesand VII improvers (other than possible make up oil, esp. during oil filter change), and also the rid of moisture and contamination of base oil...
(*I certainly cannot take on that so-called "synloob" joke out there*, obviously)
My 2c's worth.
Q.