Check this out:

http://www.lnengineering.com/oil.html

NOTE:

Shops that used to run M1 in their race cars have either switched to Mobil's synthetic motorcycle oils or have resorted to using premium dino oills, such as Swepco 306 15w40 or Brad Penn Racing 20w50 oils, for their higher levels of protection. For most owners, the reduction in longevity of a catalytic convertor is a small price to pay considering the many thousands of dollars it costs to properly rebuild a Porsche engine.

In short, be leary of "StarBurst" labelled oils, even the CJ-4 15w-40 diesel/fleet oils (e.g., Delo 400; Delvac 1300) which now say they are "emissions friendly".

Excerpt:

The purpose of proper lubrication is to provide a physical barrier (oil film) that separates moving parts reducing wear and friction. Oil also supplies cooling to critical engine components, such as bearings. Detergent oils contain dispersants, friction modifiers, anti-foam, anti-corrosion, and anti-wear additives. These detergents carry away contaminants such as wear particulates and neutralize acids that are formed by combustion byproducts and the natural breakdown of oil. Not all motor oils are created equally when it comes to the levels of additives and detergents used. The focus of this study is on the levels of zinc and phosphorus found in motor oils, more exactly, the zinc (Zn) and phosphorus (P) that makes up the anti-wear additive ZDDP, zinc dialkyl dithiosphosphate.

Many Porsche repair shops have acknowledged that these newest SM and CJ-4 motor oils are not sufficient for protecting any Porsche engine. With longevity and the protection of vital engine components in mind, many shops are recommending the addition of GM's EOS Engine Oil Supplement at every oil change. Shops that used to run M1 in their race cars have either switched to Mobil's synthetic motorcycle oils or have resorted to using premium dino oills, such as Swepco 306 15w40 or Brad Penn Racing 20w50 oils, for their higher levels of protection. For most owners, the reduction in longevity of a catalytic convertor is a small price to pay considering the many thousands of dollars it costs to properly rebuild a Porsche engine.

Oil companies have been cutting back on the use of Zn and P as anti-wear additives, and turning to alternative zinc-free (ZF) additives and ashless dispersants since Zn, P, and sulpherated ash have been found to be bad for catalytic converters. One such ZF anti-wear additive is boron. Most of the SM and CJ-4 oils we tested contain significant concentrations of boron (B) to offset the reduction of Zn and P. The performance of these zinc-free anti-wear additives has only been proven with ultra-low sulphur fuels, not readily available in the United States with exception of new diesel fuels since 2007. Since we are discussing aircooled engines specifically, the highest levels of boron we found were in Harley Davidson’s SYN3 motor oil, which is specifically formulated for an aircooled engine, but at levels six to ten times that of what is present in any reformulated SM or CJ-4 motor oil. Additionally, Harley's SYN3 didn't reduce the Zn or P, just supplemented it with the added boron. Similarly, Swepco's 306 has high levels of boron in addition to high levels of Zn and P.

However, it is worth noting that these new API guidelines do not apply to “racing,” “severe duty,” or any motor oils that do not carry an API “starburst” seal or clearly state for off-road-use only. Motor oils meeting “Energy Conserving I or II” standards should be avoided as well as those with an API SM classfication, with it's lower Zn and P levels, which applies only to 0w20, 0w30, 5w20, 5w30, and 10w30 "ILSAC" grades. Although having been more sensitive to emissions and the environment than American standards, we find the European ACEA A3 and B3/B4 classifications, which place a cap on P levels at 0.10-0.12%, to be better in taking into consideration wear and engine longevity while limiting emissions and protecting emissions control devices. Additionally, ACEA A3 sequences require higher high-temperature high-shear (HTHS) viscosities, stay in grade sheer stability, and tighter limits on evaporative loss, high temperature oxidation, and piston varnish. This makes oils meeting these ACEA standards that much better for your Porsche, especially since wear limits are much more stringent for valve train wear, 1/6th to 1/4th the wear allowed in the sequences for API's SM standards. Porsche requires a minimum 3.5 cSt @ 150C HTHS viscosity, which is a good measure of the protection any given motor oil provides, and requires that all approved oils be of group III or better base stocks, which includes quality hydrocracked parrafinic petroleum bases like those used by Brad Penn and Swepco. In general, synthetics provide the best protection, but a good additive package and quality parrafinic base stock has been proven to work just as well.

Failure to use the right oil, use proper filtration, or observe proper changing intervals can affect the performance of even the best motor oil. This also includes changing the oil too often or not often enough. Against conventional wisdom, engine wear decreases as oil ages to a certain extent, which means that changing your oil more frequently actually causes engine wear; these findings were substantiated by studies conducted by the auto manufacturers and petroleum companies, leading to drain intervals increased from 3,000mi to 5,000-7,500mi in most domestic vehicles. It has been suggested that no more than six months or 7500mi should be observed between service intervals and vehicles driven in more demanding conditions should be serviced more frequently, rather than based off of extremely long drain intervals recommended by European manufacturers, some in excess of two years and 15,000 mi. Vehicles with track time or sustained high oil temperatures or RPMs should have their oil changed after every event. Vehicles subjected to very short drives or sustained operation in heavy traffic should indeed be serviced more often. Regular used oil analysis is the best way to determine ideal drain intervals for your driving habits. With this knowledge in hand, using a quality motor oil with proper filtration and regular service is the best thing to do for your engine and to protect your investment.