I have used 15W-50 for the 9 years I have owned my Boxstir.
http://www.renntech.org/forums/index.php?showtopic=9454&hl=unapproved
I used the EP flavor for the last oil change because the regular 15W-50 had been discontinued. But since then Mobil has brought it back to the market.
While I may not be going to Porsche heaven, my 1997 runs fine.
If you don't want it send it to me.....