For the IMS geeks among you who follow ED the ex-Timken guy - he is in favor of an xW-50 oil because of load/shear/heat on the IMSB. It may be that the JGDT40 additive package offers the same benefits at a lower (40) number?
But then there is the cylinder bore scoring+cold climate issue to consider.
http://rennlist.com/forums/996-forum/904760-scored-cylinder-failure-for-your-996-y-or-n-tell-us-yr-996-mk1-or-mk2.html
So the best answer may be climate-specific with the hot climate owners having the easiest choices -JGDT40 or 10w-50 or 5W-50 if you take the risk of abandoning the M96-optimized additive package in JGDT40.Foaming is another issue.... it gets complicated