Oil Change interval thread (long end to speculation)

larry leung l.leung at juno.com
Mon Jul 9 09:15:55 EDT 2001

After reading all of the speculation, I figured I could apply some of my previous engineering education and experience to bring some of it back to reality.

We are NOT being duped by marketing. 

In addition to Mr. Selby's comments of  "normal" car operation, (so true, hard for us 'Mericans to get oils up to what auto manufacturers call "normal" operating temps that can boil off the lighter contaminants from the oil, though still leaving some of the heavier ones), the interval is based on contaminant levels in the oil. Note, one of the worst contaminants is a by product of combustion, water, which mixes with other oil contaminants to produce acids. Boil off the water, and at least the acid problem is reduced. Problem is, stateside people rarely get the oil hot enough to do this job efficiently, thus the shorter oil change interval, to remove the acid and water. Also note, the oil sold here is the same as is sold around the rest of the world. No tricks here. 

But, as far as the lower oil weights, they are reflections of new technology and engineering practices, NOT marketing. 

Oil film density and bearing clearance are dictated by the axial load at the bearing surface. This is dictated by RPM, mass of reciprocating components, BMEP (piston pressures, certainly higher in supercharged applications) and numbers and surface area of bearings. Newer machining practices have allowed closer tolerances, thus thinner oil films in the journal bearings of the engines. And, the need for better fuel economy (thus reduced HC emissions) has also altered design practices, meaning bearing surface areas have likely increased as well (through either larger diameter journals, or wider bearings). More efficient oil pumps capable of slightly higher oil pressures (with the same or less parasitic losses) would also offset the thinner oils, which do exhibit less hydrodynamic drag. In end result, it is possible to use thinner oil to do the work of higher viscosity oils by mechanical design (always has been) but better mass production machining techniques have made it possible in production cars. 

Finally, most of the improvements in oils, (i.e. the progression from SF, SG, etc. oils) has been made in the anti-contamination package, not on the lubricative properties. The largest recent step in lubricative properties was in the manufacture of some of the synthetic oil bases, whos long chain molecules are more consistant and stable than "natural" (not dino, oils are actually plant remain based) oils, allowing lesser weight oils to do the same job (i.e. dynamic film strength) as more viscous natural oils. 


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