Any information you may receive related to this commentary is provided merely as friendly suggestions, not as expert opinion, testimony or advice. 
The main 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.

Likewise, the viscosity of the motor oil throughout the operating range of the engine is very important to the "hydro-dynamic bearing" layer (oil film that forms on and between moving engine parts). Boundary Lubrication occurs when insufficient film to prevent surface contact and where the primary anti-wear additive ZDDP plays its role in protecting your engine.
Not all motor oils are created equally when it comes to the levels of additives and detergents used.
In an SAE whitepaper on the development of the API SL standard, Shell's own lubrication engineers stated that 'the introduction of ash-less and zinc free oils are on the horizon making choosing an oil that much more difficult for older engines.' 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.

Oils for modern engines have different formulation constraints than those for older engines and just because oils are "modern" or synthetic does not mean they will provide adequate protection for your engine. Shopping for oil by brand, previous reputation, or by manufacturer approvals alone does not guarantee the best oil for your engine. 
What general characteristics make motor oils specifically well suited to an aircooled engine?
Aside from recommendations issued by Porsche, what makes for a good motor oil?

These oils must be thermally stable, having a very high flashpoint, and must "maintain proper lubrication and protect vital engine components under the extreme pressure and the high temperature conditions" found in air-cooled Porsches.  Porsche recommends and uses Mobil 1® 0w40 as a factory fill and Mobil 1® 15w50 is a popular choice used by many for their air-cooled boxer engines year round in a wide range of climates.  It is worth noting that Mobil offers its own line of racing oils for track use and Porsche even now offers its own line of classic oils for protecting older air-cooled engines, so oil selection is more important now than ever. Understanding what changes have happened and why is important in selecting the right lubricant.
Oil companies have been cutting back on the use of Zn and P as anti-wear additives.  This reduction of phosphorus content is a mandate issued by API, American Petroleum Institute, who is in charge of developing standing standards for motor oils.  Zn and P have been found to be bad for catalytic converters.  In 1996, API introduced the API SJ classification to reduce these levels to a maximum of 0.10% for viscosities of 10w30 and lighter. The 15w40 and 20w50 viscosities commonly used in Porsche engines did not have a maximum phosphorus limit. The API SL standard maintained this higher limit but with reduced limits for high temperature deposits.  With the API SM, phosphorus content less than 0.08% was mandated to reduce sulfur, carbon monoxide, and hydrocarbon emissions. The biggest difference between the API SM and SN standard is that with the subsequent SN standard now mandated a max phosphorus content of 0.08% for all motor oil viscosities, not just the 10w30 and lighter oils the previous API standard limited, and limits for high temperature deposits are reduced, requiring added detergency for increased engine cleanliness allowing for longer drain intervals. Most modern oils are not backwards compatible with older engines for these reasons.
It is worth noting that prior to this movement to reduce Zn and P levels, the oils recommended for use in an air-cooled boxer engine typically had 0.14% Zn and 0.14% P content with less detergency, than current street car formulations. In comparison, an API SE-rated virgin oil sample of Kendal GT-1 motor oil from the 70's, pre-dating today's limited Zn and P mandates, contained 0.14% Zn and 0.12% P and significantly reduced detergency with the relatively short drain intervals then recommended by auto manufacturers.

Oils with later API SH and SJ standards with no limit for phosphorus were developed, tested, and used in air-cooled engines through the end of production of the Porsche 993 with air-cooled Mezger engine. With this knowledge, it can be concluded that any given motor oil should have a minimum of 0.14% zinc and 0.12% phosphorus for aircooled engines.
To offset the reduction of zinc and phosphorus levels required by the EPA, boron as well as molybdenum disulfide, among other friction modifiers, has been added to modern oils. The addition of boron, when in the presence of ZDDP, does boost the anti-wear properties and although considered an anti-wear additive, the use of "moly" has been truly limited only to increasing fuel economy requirements of the CAFE (Corporate Average Fuel Economy, enacted by Congress in 1975). These additions do not completely address wear issues of older vehicles that require higher levels of Zn and P.
However, it is worth noting that updated 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, except those oils carrying an API SN rating, which should all be avoided.  Motor oils meeting "Energy Conserving I or II" standards, that provide emission system protection, or extended drain intervals should be avoided.  Conventional 10w40 and 5w50 grades, because of their lack of shear-stability and relatively high amount of viscosity improvers, should also be avoided.
Compared to conventional oils, synthetics have superior shear stability leading to improved resistance to thinning and evaporation at high temperatures

Synthetics also have superior cold flow characteristics, reducing start-up wear significantly. Although most modern synthetics incorporate seal swelling agents, for those concerned with formation of new leaks or worsening of existing leaks, an acceptable compromise is the use of conventional, semi-synthetic, or group III synthetic (as compared to group IV and V synthetics), which is formulated from very highly refined "hydro-cracked" petroleum base with synthetic additives. Regardless of your choice to use conventional or synthetic lubricants, the formulation is just as important as whether it is a non-synthetic or synthetic oil.

Lastly, other than cost, there is no reason not to use a synthetic oil in your Porsche or any other air-cooled engine.
Coupled with reduced oil viscosities, modern engine oils are designed to maximize fuel economy, extend catalytic converter life, and reduce tailpipe emissions. It is more important now than ever to select the right engine oil. Failure to use the right oil, use proper filtration, or observe proper changing intervals can affect the performance of even the best motor oil. With this knowledge in hand, using a quality motor oil with proper filtration and reduced drain intervals, as recommended by your Porsche mechanic, is the best thing to do for your engine and to protect your investment.

Charles L Navarro
LN Engineering LLC
125 Gladiolus St. | PO Box 401 | Momence, IL 60954 | Phone (815) 472-2939 Fax (413) 280-9041
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