[Normy]

Your Aztec's O-540's require a straight-weight for the same reason that my lawnmower does: these engines primarily work within a small rpm band.

Aircraft engines are required to make mazimum power for long periods of time, and generally have a required warm up time. If you consult the AOM for your Aztec I'm sure that it will tell you not to apply takeoff power until the oil temperature reaches the green band. Normally, that's before you even reach the run-up area, but on a cold morning in Michigan then you might have to wait for a little while.

The reason for this is that cold oil doesn't flow, it tends to build pressure. The oil pump produces pressure that is mechanically demanded- that is, the oil is forced into the bearings. If oil flow is blocked, since oil is not compressible, then either the pressure relief valve will open or one of your oil lines will explode. Since the bearings in 9.0 liter air-cooled flat six engines tend to be built with generous tolerances, it only takes a few minutes of idle operation to reach "green band" temperatures. But here's one thing your Porsche will probably never experience: Full power for several minutes, then continuous 75% power for several hours at continuous rpm. THAT situation requires a thick oil- I think that most aviation engines require straight 50 weight ashless-dispersant oil after the first 50 hours in order to provide sufficient oil pressure at high temperatures under these conditions.

Your Porsche might be driven hard, but it doesn't see that sort of use! That is why you need to use the oil that is recommended in the owners manual in both your 928 and your Aztec. For the Porsche, I recommend Mobil Delvac 1300 in 15W40; Doug Hillary over on Rennlist has done an AMAZING job reporting on all of his oil condition reports, straight from the laboratory, while running this stuff for upwards of 14,000 miles.

Best of luck!

N