quote:
Originally posted by MikeW:
Given the parameters: A location that has real winters, and a fairly non-stressed SUV engine, engine using a significant amount of oil but having no other noticable problems, and that 5w30 is the mfgr spec'ed vis (I never got this as a fact, but I assuming it. If it's wrong, just substitute the spec'ed vis)...
When the engine is warm and the oil is well distributed at the proper pressure, and operating in a low stress enviroment, an oil with the spec'ed vis should be keeping all the bearing surfaces separated, and the engine would be experiencing essentially no wear. Even normal stop and go driving would fall in this scenario. Upping the vis in this circumstance buys you nothing. It might even be costing you something, by reducing oil flow. In other circumstances (towing, high performance, extreme heat, etc), upping the vis might help.
Most wear comes at startup (engine cold, oil cold, at its highest viscosity, and down in the pan). You want to get oil flowing ASAP. The argues for the lowest w rating that's in range for the engine. Given this, you COULD go with a super wide range multigrade synth, but, as explained above, having the increased high temp vis doesn't buy you anything, and the oil is more expensive.
Now remember the engine is using oil, so you really don't want to be using expensive oil, since it will just burn it anyway.
Hence my suggestion
Mikey,
Not to be an @ss, but you must understand that oil is simply more than just "oil"...there's a lot of other stuff in the mixture, but for this discussion, anti-wear and extreme pressure additives are the most crucial.
In regards to wear while the engine is running, the oil film is always being sheared away or squeezed away in an engine, most notable in regards to areas such as the top rod bearing, around the rings, and the cam/lifter interface. Your engine does wear, but it is the additive package that actually stops most of the wear in crucial areas. Upping the viscosity will place a thicker hydrodynamic film between various parts and place a lower importance on the mixed-film/boundary lubrication properties of the oil.
Very similar is what I have to say in regards to cold-startup wear.
Once again, if it were just plain oil with no additives, then on cold-start up's, your engine would definitely wear at a high rate, and the use of the lightest oil would be of the upmost importance.
But, additives like molybdenum, phosphorus, and boron supposedly chemically react with the metal and form strong films. On startup, the film protects and metal on metal action until a fluid film is established.
Now, yes, you are right, a thin oil would be good, as it would place less reliance on the additive on startup.
With that said though, I'm running Delo 400 15w-40 in my Chevy pickup/4.3L V-6, and oil pressure is established almost immediately to about 33psi, so I'm not the least bit concerned with running a thinner oil.
Winter weather could change my mind...we'll see!
[ September 19, 2003, 12:51 AM: Message edited by: Jelly ]