Change in oil additives?

[Duke W.]

Re: Change in oil additives?

I'm still researching and trying to find out "the deal" on SM oil, but the indications I have are that ZDDP has been vastly reduced or maybe even eliminated from SM. It may take some time for me (or others, perhaps) to fully sort it out.

I have not seen a CI-4 or CI-4 Plus that meets SM. SL is as high as it goes for SI service categories, and my current hunch is that CI-4s will not meet SM because they have too much ZDDP.

OHC designs generally place lower loads on sliding surfaces because they require little valve spring force compared to a pushrod design. I can just barely open the valves on my Cosworth Vega with both thumbs and a concerted effort, and the limiting valvetrain speed is lofty 9000 even with the modest valve spring force. Also, many SOHC/DOHC engines are moving toward roller rockers. The GM Ecotec is an example.

The one critical boundary lubrication condition that both modern and vintage engines share is piston/ring reversal where instantaneous relative velocity reaches zero, which precludes hydrodyamic lubrication.

The SM oil question is somewhat moot for me as I am now running CI-4 in all my cars (the newest of which is a '91), and I will avoid SM until I can find out more about it.

Duke

[Duke W.]

Re: Change in oil additives?

The first SAE viscosity grade such as 10W in a 10W-30 establishes the range of absolute viscosity at 32F and the second rating means the viscosity is within the range of a SAE 30 at 212F. Like Joe said, to achieve this dual rating, you start with a SAE 10W base and then add VI (viscosity index) improvers so that the oil doesn't loose viscosity with temperature like a straight 10W and meets the viscosity range of a 30 at 212F. Effectively, a 10W-30 doesn't thin out as much as a straight 10W as temperature increases, so you get the benefit of easier pumping when the engine is cold and good film strength (higher absolute viscosity) when the oil is hot.

Early multivis oil VI improvers "wore out" relatively early due to both shear and oxidation, but modern VI improvers are much more stable so a mineral base oil will maintain its original multivis characterisitcs much longer.

Most synthetic base blends have a better "natural" viscosity index, so they need less VI improver than a mineral oil base and sometimes no VI improver depending on the base blend and the desired viscosity range.

Duke

[George J.]

Thanks Joe and Duke *NM* *NM*