what oil to use

You can use diesel oil but not for much longer. The reason that the zinc levels were reduced is that zinc supposedly causes deterioration of the catalytic and since diesels are getting cat's all of the diesel oils will soon lose their zinc as well. This most likely means Rotella, Delvac, Delo, Mopar, all of them. And you will not know when that happens. The only oils I know of that are good for sure are Royal Purple, any of Amsoil's synthetics, Valvoline VR1, and Redline. E-Mail the manufacturer of whatever you like to use and ask them for the ZDDP concentration. If it is lower than 1200ppm or if they won't tell you, stop using it and bite the bullet for the good stuff. I use Amsoil Formula 2000 20W-50 Severe Service Racing Oil. After reading the charts and seeing that it's viscosity (cold and hot), volitility, flash point, and just about everything else you care to name was better than everyone else, I read tons of real-world reviews on various racing forums and could not find a single bad review of Amsoil oils other than whining about the price. I even saw several posts claiming slightly quicker ET's and faster turbo spool-up times. The only thing I noticed was that the engine turns over a little faster when cold.