You probably already know this but buy -C-
rated oil, not "S".The "C" specification is for diesel (compression ignition) and the "S" series is for gasoline (spark ignition). Within each category is a series of specifications like CC, CD, CE, CF, CG, CH, SG, SH, SJ, etc. Look at the API website for better info. If you just want the short version, this was from Mobil's website: Here is a short description of the diesel engine categories, starting with the most current.CH-4 is a proposed category at this time, which we anticipate being finalized during the first half of 1999. CH-4 is being developed to address engine design changes required to meet new 1998 EPA emission standards. CG-4 was developed for oils used in 1994 and later, low-emission, high-speed, four-stroke cycle diesel engines. It is also typically used for diesel engines using low-sulfur diesel fuel. Newer engines that are used on-highway or off-highway applications may call for CG-4 quality oils. One of the main advances in CG-4 versus previous quality levels is the ability of the oil to handle high levels of soot that is generated by some of these low-emission engines.CF-4 was established in 1991 to specify oil quality for severe- duty, low-emission, four-stroke diesel engines produced in 1991 or later. Oils meeting this category offered improved control of oil consumption and piston deposits versus previous quality levels. CF-4 has essentially replaced the CE quality rating.CF-2 was established at the same time to specify oil quality for severe duty two-stroke diesel engines. CF-2 has essentially replaced CD II.CF was established for service typical of off-road, heavy-duty diesel engines and engines operating on higher sulfur fuels. The CF quality level essentially replaced the CD level.CE was established in 1988 to specify oils for severe-duty diesel engines with turbochargers. This rating is essentially obsolete and oils meeting CF-4 are now used in applications calling for CE oils. etc, etc.I think "CI" rating is the latest certification....but I think any of the deisel oils above CE would be fine for the venerable Universal 25XP....the high in the "alphabet" the better the engine oil should be engineer (relatively speaking)...here is the new CI Oil specshttp://www.infineum.com/information/heavyduty2.htmlHere is an "older" API flyer on grade of oil http://api-ep.api.org/filelibrary/ACF28.pdfok....with all that said -- just stick to one good brand (I like shell rotella) and change it often and you should be fine. I only use a single weight (30W) and its fine ...you don't have to have a multi-weight and there are arguments against it...but for our application any good brand oils will be fine.Here is some interesting and informative readinghttp://www.vtr.org/maintain/oil-overview.htmlMore trivia -- For oil listings start with 'S' people usually think "spark" but really its meaning is "Service" category, but most just think Spark-plug ignition For diesel oils, the first letter is 'C' and most think it is officially "Compression ignition" -- but it really comes from the "C" in "Commercial" category.http://www.chris-longhurst.com/carbibles/index.html?menu.html&engineoil_bible.html