I stumbled upon an interesting new post by Jim Morrissey of Royal Purple in the epic Ultimate Motor Oil Thread thread on the Pelican Parts Technical BBS. Jim discusses some formulation changes in Royal Purple oils that may affect owners of older Porsches who depend on higher ZDDP levels for engine longevity.
I just wanted to pass on some information related to this thread topic since we (Royal Purple) recently reformulated our SAE/API street oils and although both old and new formula are still often able to be found, a little checking of the back label for the API license will be worth doing.
Due to the reduction in anti wear from the API-SM cap of 800ppm phosphorus our SAE/API-SM and SN street oils are no longer the best choice for flat tappet performance engines with lifts above .500". We do not recommend our SAE/API-SN 20W-50 (part #01250, quart bottle) or SAE/API-SN 10W-40 (part #01140, quart bottle) for those applications any longer.
You can go straight to the post for the details of which specific formulations have changed. I'd also recommend checking back with this thread for further discussion on what's bound to be a controversial topic.
If you want some background on why ZDDP levels are important for Porsches in particular and older cars in general — some would argue all cars — go back to the first post in that Ultimate Motor Oil Thread and follow the links.
Hats off to Royal Purple for being up-front about the formulation change and bringing a clear explanation to the folks most likely to be affected.