Delvac 1300 or Castrol oil?

[DieselHead]

I just got back from our local warehouse store and while they have always sold Castrol and Mobil oil, they just recently started to stock 4 gallon cases of Mobil Delvac 1300 15W40 oil. I read the bottle and it sounded pretty good, stating that it was industrial grade, and listing all sorts of farm machinery and diesel engines it was recommended for. So, my questions is, for my next oil change (coming up soon), should I go with Castrol 20W50 (or 10W40, they have all grades) or try this Delvac stuff? The Delvac 1300 isn't that much more expensive. The Castrol is $18 for 12 quart bottles, and the Delvac was $22 for 4 gallon containers. Thanks for the advice

Also, Anyone know anything about "Power Service" diesel fuel supplement? Someone bought a huge container of it for me (actually, it was my girlfriend, she knows how much I love this car) and I don'tknow whether or not it's worth using. The STP stuff I've been using (only about once a month or so) hasn't proven to be the greatest thing ever, and no one sells the Redline stuff around here (I'll have to order it from the PartsShop when I get a chance).

Alex