After a lot of reading over 18 months, including a period of suffering ZDDP anxiety and realising that 99.5% of what is posted on internet forums is not based on fact, I posted the following on my local Porsche forum to alleviate the anxiety that I know many of my fellow enthusiasts suffer. Hope this helps!
Engine Oil for Air Cooled 911’s – Select in 3 Easy Steps
The other day I was asked by someone who only has a passing interest in engine oil (can you imagine that!) on how they should select and offer the following suggestion:
1) Drive to your favourite auto parts store.
There is no need to special order oil over the internet and pay high shipping for some oil that has internet folk law, just go to your local store or service station.
2) Choose your viscosity grade preference.
For our purposes, something between 5W-40 and 20W-50 will be fine.
Don’t waste sleep on this as it doesn’t materially matter as we will neither live long enough nor have enough objective data to be definitive in our choice.
3) Porsche A40 Approved or a Suitable API/ACEA Service Classification?
Your Driver Manual probably references API service classification like SH, SJ, SL or similar which was all that was available at the time and have been long superseded by much better oils.
If money is no object, just buy 10 litres of Porsche A40 approved oil. You pay a premium for this but if anyone knows best, it has to be the manufacturer. However the likely number of oils on the shop shelves that have this approval will be small, will take some reading of labels to identify these few and you will pay a substantial premium for this comfort.
However if you can’t see the value in using a Porsche approved oil, I suggest choosing an oil that meets ACEA specification A3/B4. Why? The ACEA A3/B4 approval will guarantee a High Temperature High Shear (HTHS) of greater than 3.5 mPa.s just like Porsche themselves specify and the relatively high ash (SAPS) limit of 1.6% does not adversely limit the use of anti-wear additives. There are other good reasons too but that's for another time.
Using the ACEA rather than the API classifications also makes it simpler in identifying suitable oils because of this direct correlation between what Porsche specify and what the A3/B4 specification delivers.
Applying the above gives plenty of choice