Oil – What motor oil is best for my aircooled Porsche? (or any high performance engine)

The purpose of proper lubrication is to provide a physical barrier (oil film) that separates moving parts reducing wear and friction, but there are many surfaces within an engine that operate with metal-to-metal contact, again popular belief, that are very highly dependant on a strong and robust anti-wear film. The top piston ring operating in sliding contact with the bore operates in a mixed lubrication regime consisting of both boundary (metal-to-metal direct contact) as well as hydrodynamic (oil film between moving surfaces) lubrication. The majority of non-corrosive wear occurs where boundary lubrication exists, especially at cam lobes, tappets, cam follower/buckets, and rockers. Oils contain dispersants, friction modifiers, viscosity modifiers, anti-foam, anti-corrosion, antioxidant and anti-wear additives, all of which can affect the strength and durability of anti-wear films. The focus of this study is on the levels of zinc and phosphorus found in motor oils and their interactions with other additives, more exactly, the zinc (Zn) and phosphorus (P) that makes up the anti-wear additive ZDDP, zinc dialkyl dithiosphosphate, as the ZDDP level is causing concern for all older engines, including aircooled Porsches, with modern oils.


What general characteristics make motor oils specifically well suited to an aircooled or other high performance engine? Aside from recommendations issued by Porsche, what makes a good oil? These oils must be thermally stable, having a very high flashpoint, low noack volatility, and must “maintain proper lubrication and protect vital engine components under the extreme pressure and the high temperature conditions” found in aircooled Porsches. Many well-known Porsche engine builders recommend 15w40 viscosities below 90F ambient air temperatures with 20w50 for hotter climates above 90F average ambient temperatures in street use. Porsche recommends and uses Mobil 1 0w40 as a factory fill in new vehicles and Mobil 1 15w50 has been a popular choice used by many year-round in aircooled Porsche models. What was once considered a ‘safe’ oil is no longer as many of these lubricants have been reformulated for many reasons, not limited to allow for protection of emissions controls and for longer drain intervals and shopping by brand alone no longer ensures satisfactory performance.


Even prior to the introduction of the API’s SM and now current SN standards, there was concern that current API SL standards from back in 2003 may inhibit the backwards compatibility of motor oils, specifically referring to the limitation of ZDDP, which is “the most effective combined anti-wear and anti-oxidant additives currently available.” SAE 2003-01-1957, Effect of Oil Drain Interval on Crankcase Lubricant Quality, Shell Global Solutions. The authors continue to state that oils are required to provide longer protection in severe operation but that an oils performance is “limited by environmental considerations.” Furthermore, they state that it is hard to predict the effects of these reformulated oils in just a single oil change and may only be evident over an engine’s lifetime. It is hard to know the full extent of the potential damage these new SM oils will have on our performance engines so chose your lubricants carefully.


Porsche’s recommendation in hand, our initial analysis from 2005 and 2006 and from virgin oil analyses going back to the 1990s, we found that then prior SH/SJ formulations of Mobil lubricants tested, including Mobil 1, have had higher Zn and P content than SL, SM, or current SN formulations. Even current “re-introduced” formulations are not the original formulations many shops and owners were used to. Aside from reduced Zn and P levels (now restored in certain products), many products with “adequate” Zn and P still use high levels of Ca detergents, well documented in various SAE publications as known for causing more wear than Ca/Mg or Ca/Mg/Na detergents, as previously used in oils like Mobil 1 15w50, back when it was API SH/SJ rated and prior to reformulation. This confirms the industry wide trend of the reduction of Zn and P from motor oils and switch to Ca-based detergents, with the eventual reduction to 0.06-0.08% or even worse, the elimination of these additives, which are essential to an aircooled Porsche engine’s longevity. Depending on how detergent an oil is and which detergents are used, optimal Zn and P levels can range from 1200 to 1500 ppm, lower detergency oils requiring less Zn and P.


Many Porsche repair shops have acknowledged that these newest SN and CJ-4 rated motor oils are not sufficient for protecting any Porsche engine, including newer water-cooled ones. With longevity and the protection of vital engine components in mind, many shops are recommending non-approved motorcycle or racing oils, or the addition of oil supplements at every oil change, for their higher levels of protection. On newer water-cooled Porsche engines where Mobil 1 0w40 is recommended, a simple change to an oil with CJ-4 rating or preferably an oil with SL or CI-4 rating as well as a viscosity of 5w40 rather than 0w40 are two changes that can be done in addition to more frequent oil changes to ensure longevity of newer engines. Granted most of these oils will not come with Porsche approval, so if your vehicle is under warranty, defer to the Porsche approval list and select a 5w40 viscosity where appropriate over that of a 0w40, regardless of API rating, as all will carry the most recent API ratings.


Oil companies have been cutting back on the use of Zn and P as anti-wear additives and switching to alternative zinc-free (ZF) additives and ash-less dispersants in their new low SAPS oils since Zn, P, and sulfated ash have been found to be bad for catalytic converters. One such ZF dispersant/anti-wear additive is boron, which does not foul the catalysts in the particulate emissions filters or catalytic converters. For most owners, the reduction in longevity of a catalytic converter is a small price to pay considering the many thousands of dollars it costs to properly rebuild a Porsche engine. It is worth noting that most Porsches have lived the majority of their lives with high Zn and P oils as found in API SG-SJ oils as late as 2004, and we never hear of problems with their catalytic converters.


In addition to protecting emissions controls, there are many other design considerations in formulating engine lubricants, which include improving fuel economy and longer drain intervals. Many believe that the EPA has banned zinc and phosphorus in motor oils. This is not true. In response to modern engine design and longer emission control warranties which are required by the EPA, manufacturers have turned to reformulation of oils to do this, as well as to improve fuel economy by reducing fiction. High friction can result in areas with boundary lubrication or where high viscous friction forces and drag may occur with hydrodynamic lubrication in bearings. The use of friction modifiers, such as moly (there are many different species of Mo-based friction modifiers, help to reduce friction in metal-to-metal contact with the formation of tribofilms characterized with their glassy, slippery surfaces. Lower viscosity motor oils are key to increasing fuel economy by their reduction in drag where high viscous friction occurs in hydrodynamic lubrication. While lower viscosities improve fuel economy greatly, they also reduce the hydrodynamic film strength and high temperature high shear viscosity of the motor oil, factors both of which are key to protecting high performance engines, especially aircooled ones. It is worth mentioning that lower viscosities will provide better fuel economy, but thicker oils in modern engines greater than what the manufacturer recommends and what the engine is built for will not always result in better protection.


However, it is worth noting that these new API guidelines do not need 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 or ILSAC GF-4 classifications. The European ACEA A3/B3 “mid-SAPS” classifications, which place a cap on P levels at 0.10-0.12% but allow for higher Zn levels, to be better in taking into consideration wear and engine longevity, setting much lower wear limits, while still limiting emissions and protecting emissions control devices. A good example of this is a Porsche approved lubricant – although it may carry an API SM or SN rating, it will by rule require 0.10-0.12% Zn to meet the ACEA requirements. The current ACEA A3/B3 classifications require higher high-temperature high-shear (HTHS) viscosities, stay in grade sheer stability, and tighter limits on evaporative loss (noack volatility), 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 or CJ-4 standards. Of particular interest is the upcoming ACEA E9 which will incorporate some of the improvements in the CJ-4 standard along with higher Zn and P, making these mid-SAPS oils an excellent choice for older legacy engines.


Failure to use the right oil, use proper filtration, or observe proper changing intervals can affect the performance of even the best motor oil. It is also worth noting that some manufacturers have gone to shorter intervals and requiring fully synthetic oils (Group 4 or 5) due to litigation surrounding sludge formation and failed engines as a result of factory recommended long drain intervals, so drain interval recommendations are often in a state of flux. Based off of extremely long drain intervals recommended by most European manufacturers, some in excess of 30,000 mi at some point in recent history. Most users have found it best to reduce those intervals by half or even a quarter. Porsche over the last decade has had intervals ranging from 12,000 to 24,000 miles and up to 2 years. Based on UOAs provided to us by our customers, new Porsche owners should consider reducing their drain intervals to no more than six months or 5,000 mi. On newer water cooled models prone to IMS failures, some shops even recommend changing your oil every three months or 3,000 mi allowing for early detection of a failing IMS bearing. Also, remember, Porsche drain intervals are based off a fill of min. 10 quarts, so engines with smaller sumps have to run shorter drain intervals! Cars that have less oil volume can benefit from more frequent oil changes.


Vehicles that are driven spirited or on the track, subject to sustained high oil temperatures or RPMs should have their oil changed more frequently or in the case of cars used at the track, the oil should be changed after every event (or every other event). Likewise, vehicles subjected to very short drives or sustained operation in heavy traffic should indeed be serviced more often. Vehicles not driven often but driven hard a few times a year can probably go a year between oil changes as long as the oil is changed before the car is put away for winter for storage. Regular used oil analysis is the best way to determine ideal drain intervals for your driving habits – one good rule of thumb I have seen quoted is to change the oil with the TBN (total base number) is reduced by 50% of the original total (requiring you to also know your oil’s virgin TBN). Another common recommendation is to change the oil once it’s TAN (total acid number) equals the TBN. Other factors to consider are fuel dilution and shearing out of grade when determining your drain interval. 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.


Any information you may receive related to this web site is provided merely as friendly suggestions, not as expert opinion, testimony or advice. Neither LN Engineering nor Charles Navarro endorses or sponsors any information, products or methodologies you may find herein. – Sorry for the legal mumbo-jumbo!