First problem;
Polyalpha olefin (PAO), is the synthetic basestock used in AeroShell 15W-50 (at 50%) and Exxon Elite (at 26%) and the defunct Mobil AV1 (at 100%). It has excellent high and low temperature viscometric properties, high viscosity index (doesn't’t thin as much with increasing temp) and (low temp pour point) and good high temperature stability (when used with the proper antioxidant package). However, NONE OF THIS IS IMPORTANT FOR AIR-COOLED AIRCRAFT ENGINES! PAO has terrible solvency characteristics. It is so bad, that most additives will not dissolve in it. It needs to be combined with an ester (10-20%), alkylated naphthalene (5-25%) or mineral basestock (40-75% Aeroshell and Elite) just to get the additives to dissolve. This is fine for a heavily additized passenger car motor oil but NOT for a low additive treat rate oil used in a very high blow-by, leaded fuel aircraft engine. The ability to keep an engine clean by keeping combustion by-products in suspension is essential for an aircraft oil and the basestock works hand in hand with the dispersant to achieve this.
In my opinion PAO is the worst possible choice of basestock for piston aviation oils, and Exxon and Shell did not learn anything from Mobil’s AV1 spectacular failure. Mineral oils (non dispersant) by themselves, have difficulty solubizing the blow-by for long (witness the engine varnish with the use of non-dispersant oils) but it is nothing compared to the problem PAO has with it. The problem with Mobil AV1 was never the lead bromide (lead salt) particles, it has always been the partially combusted blow-by fuel in the crankcase that forms resinous varnish and captures the lead particles making a thicker deposit that is the problem. It was that way with Mobil AV1 and it remains so with the semi-synthetics.
The second problem;
There are two phosphate anti-scuff additives that meet the requirements of AD-80-04-03, and they can be used interchangeably with both of them having been sold as LW16702. One is butylated triphenyl phosphate (bTPP) and is currently used in the AeroShell 15W-50, W100 Plus and the Lycoming LW16702. The other is tricresyl phosphate (TCP) or methylated triphenyl phosphate, and it is used in the Exxon Elite. While they are used interchangeably to satisfy the AD, they act very differently in an engine.
bTPP, used by Shell and Lycoming, is a moderately good anti-wear owing to its ready decomposition and formation of sacrificial phosphate films on the surfaces of cams and lifters. The problem is that bTPP ALSO decomposes another way, in the presence of heat, water and metal all found in an engine crankcase. This decomposition is called hydrolysis and the breakdown products are oil soluble phosphoric acid derivatives that are corrosive to copper and elastomer seals. When Shell changed from TCP to bTPP in the 90’s people freaked that the copper levels in their oil analysis shot up. Shell responded by putting additional copper corrosion inhibitor in the oil. This protected the copper but left the offending acid products in the oil which left the seals vulnerable to attack. High silicon numbers in an oil analysis can be caused by the acid attack on silicone seals. Push-rod tube seals and valve cover gaskets are often silicone and prone to weeping problems. While no means a catastrophic situation, weeping seals, increased silicon in oil analysis reports, Retarded timing changes on Lycomings (from magneto pad degradation) have all been observed by the public, Shell and Lycoming.
Shell and Lycoming also say it is the addition of the Lycoming additive, bTTP, to oils already containing bTPP (AeroShell) that’s their story and their sticking to it.
bTTP is also a good friction modifier that can make marginal Continental adapters slip. It is NOT the PAO synthetic basestock that causes the slippage.
TCP, used in the Exxon oil, on the other hand is a much more stable molecule and as such, is completely ineffective as an anti-scuff/anti-wear. TCP is often used as an Extreme Pressure (EP) additive in grease but is never used as a primary anti-wear. Anti-wear conditions are much milder than EP conditions. For example, put TCP in an automotive engine as the only anti-wear, and the cam will quickly fail.
Ed
