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Modern Oils and the Air-Cooled Engine

September 27, 2010 | Troubleshooting & Repair

By Charles L. Navarro
Provided courtesy of LN Engineering

Summary: What oil should I use in my 356?

[Ed: The question of "What oil should I use in my 356?" is often asked. Below is what Charles Navarro suggests, depending on whether you want to use traditional oil or a synthetic formulation.]

When in doubt, look at the label. Do not use an API SM or API CJ-4 motor oil in your Porsche 356. Also, it is best to use an oil that does not need supplemental additives!

Inexpensive Dino Oil: Kendall GT 20w50 or Castrol GTX 20w50 (or it's High Mileage formulation). I'm not a big fan of GTX, but if changed often, it's okay.

Good Dino Oil: Valvoline 4-stroke motorcycle oil 10w40 or 20w50. This oil is easy to find and comes with an API SF/SG/SJ rating.

Best Non-Synthetic Dino Oil: Swepco 15w40 "306" motor oil. Many shops who had been using Mobil 1 have switched to this and most if not all their problems (including bearing failures) went away.

Best Semi-Synthetic:Brad Penn Racing (it's 100% Pennsylvania crude and a modern version of the green Kendall GT from decades past) and has 10% group III synthetic blended in for cold start and extreme heat protection. Regardless of it's name, this is a street oil.

Synthetic: Mobil 1 V-Twin 20w50, Royal Purple Max Cycle 20w50, or Amsoil Harley V-Twin 20w50. The Mobil 1 motorcycle and Royal Purple Max Cycle oils are also available in 10w40 viscosities. If I was going to be racing, I would use one of these oils or for a paraffinic based oil, the Brad Penn or Swepco oils.

Oil Weight: I do not think any 356 should run anything lighter than a 15w40 IMHO, unless it is being driven in the winter and cold started below 10-20F.

Break-In Oil: Brad Penn Racing Break-in oil. It's cheap insurance.

Oil Additives: EOS is now available again from your local GM dealership. If added to Mobil 1 15w50 (not the extended performance version), for example, one bottle of EOS will last approximately three oil changes. Red STP 4-Cylinder Treatment will also work — one bottle to each oil change. That said, it's best to use an oil that is properly formulated from the start and doesn't require supplementation.

The Details

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.

What general characteristics make motor oils specifically well suited to an aircooled engine? Aside from recommendations issued by Porsche, what makes a good 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 aircooled Porsches. Porsche recommends and uses Mobil 1 0w40 as a factory fill in newer watercooled models and their 15w50 has been a popular choice used by many for their aircooled models year round in a wide range of climates.

Porsche's recommendation in hand, our initial analysis from 2005 and 2006 found that then recent SH/SJ/SL formulations of Mobil lubricants tested, including Mobil 1, have had similar 0.12-0.14% Zn and P content, which we thought was a good thing, but looking closer, Mobil 1 0w40 had somewhat less Zn and P, at 0.10%. Current SM formulations are at the 0.10% level or less. This confirms the industry wide trend of the reduction of Zn and P from motor oils, 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.

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 non-approved oils or the addition of oil supplements 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 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, and we never hear of problems with their catalytic converters.

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 sulfated ash have been found to be bad for catalytic converters. One such ZF dispersant/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.

Additionally, Boron works best in the presence of Zn and P and may better serve to complement these anti-wear additives than as a replacement for them, according to recent studies. Since we are discussing boron and 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. 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.

This reduction is a mandate issued by API, American Petroleum Institute, who is in charge of developing standing standards for motor oils. The latest API SM standard for car oils calls for a zinc and phosphorus content less than 0.08% to reduce sulfur, carbon monoxide, and hydrocarbon emissions. As a result of this mandate, some motor oils now have as little as 0.05% zinc and phosphorus. Prior to the new CJ-4 API standard for diesel oils, we found most of the CI-4 15w40 and 5w40 oils to have excellent levels of Zn and P. We did observe Mobil, among other manufacturers, beginning the use of boron in their oils as a zinc-free (ZF) anti-wear additive in various CI-4 and SL formulations, but always with high Zn and P levels, above current API limitations.

It would appear now that with the reduction of Zn and P in these newest CJ-4 oils, that boron will now become a more common anti-wear additive, and even with lowered Zn and P levels, the boron levels are still nowhere as close to what previous CI-4 and SL oils, so the long-term performance of these new oils is unknown and unproven in vehicles running fuels other than those classified as ultra-low sulphur, typically less than 10ppm as alluded to earlier. Remember, unleaded fuels don't have these low sulphur levels, at least right now anyways!

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 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.

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 newest SM or CJ-4 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 highly refined 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 according to research presented to the SAE.

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 (needlessly bad for the environment and your wallet) 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, using mostly non-synthetic oils.

Based off of extremely long drain intervals recommended by most European manufacturers, some in excess of two years and 20,000 mi, some users have found it best to reduce those intervals by half or even a quarter. 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 — a good rule of thumb is to change the oil with the TBN, or total base number, is reduced by 50% of the original total (requiring you to also know your oil's virgin TBN). 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.

For more information and oil sample results, please visit

Any information you may receive related to this article 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.