Re: O.T. 15W40

Posted by txblu on June 08, 2004 at 08:20:31 from (199.46.199.231):

In Reply to: O.T. 15W40 posted by rustyfarmall on June 07, 2004 at 19:21:47:

Things you don't discuss: Religion, politics, oil............... Grin.

I am an oil pressure nut. I have a worn out tractor that I just bought. I don't know what oil it had in it when purchased. But I do know this.

Start it up cold and the oil pressure is around 55 psig at idle. Mow with it for an hour and the pressure is 5 psig at idle.

I changed to Mobil 15W-40 C-4 oil. Pressure under same conditions held at 10 psig. Went to the store for more oil.

Changed again using HD 40 wt this time. Pressure held at 15 psig which is redline on the oil gauge.

So my question is this: If I only have 5 psig putting oil between two metal surfaces in the engine, how hard is it going to be for the engine to squish that oil out and attempt to put metal on metal.

I agree with film strength, but I'd bet modern oils have approximatly equal film strengths and molecular size................except synthetic. I have proven to myself that not only does it (syn)have a higher film strength, it also adheres to metal longer that paraffin....i.e. initial lubrication at start up indicated by noises the engines make or don't make including stopping piston slap in the winter time on cold starts of my diesel engines.

Residual oil film is what protects engines until new oil is circulated at engine start regardless of how long it takes to get there (due to viscosity). So if all non syn HD oils have approximate equal film strengths, the other consideration is the ability to retain this protective film under heavy loads and that is what viscous oils that produce high oil pressure do.

So, other than this tractor, I use Mobil 15w-40 in everything and Mobil 1 15w-50, 100% when I can afford it, 20% usually, and only on my oldest tractors, 0%.

The reason the oil companies recommend light oils is they have EPA mileage quotas to meet and it doesn't take a rocket scientist to realize that a crankshaft splashing around in thick oil takes more hp than one splashing around in thin oil.

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