The following from an MG enthusiast website is a little long, but raises some questions about compatibility of modern oil formulas with older flat tappet engines. It may be an issue with our older engines, particularly cams and transmissions. I think I'll go buy a case of 20W-50 diesel motor oil, just in case.
This has been covered here: http://www.jalopyjournal.com/forum/showthread.php?t=138386&highlight=cam+break+in ANd here: http://www.jalopyjournal.com/forum/showthread.php?t=131246&highlight=cam+break+in
I got bad news for you guys. The new diesel formulations are going to start going down hill too. The new 2007 diesel emissions standards requre new oil and the old oil will be being phased out.
Note that only the "Made in Germany" Castrol had high ZDDP. the US or any other non-german Castrol is a lower ZDDP formula. The bottle will say "made in Germany" in teh fine print on the back.
use a 15W40 "CC" rated oil (Wal-Mart, etc.) or motorcycle oil and a bottle of GM's EOS (engine oil supplement, available at GM dealer parts dept's) I sell oil & lubricants and specialize in applications for extreme enviornments. www.oldcarfart.com/Amsoil