The king of austenitic stainless steel, 904L, has exceptionally high corrosion resistance, as it's most often used in piping for heat, pollution control, and bleaching equipment. 904L contains more chromium, molybdenum, and nickel than 316L stainless and amounts of silicon, copper, and manganese. These additions enhance the material's anti-corrosive properties and give it a superior polish and shine.
Rolex began using 904L in its SeaDweller watches in the 1980s and has since made this grade of metal standard in all of its watches. Today, several luxury watchmakers use 904L in their timepieces, but Rolex repair gurus generally agree that Rolex has been the most successful at marketing the grade as a "precious metal." The Swiss watch icon even created its own patented 904L alloy called Oystersteel.
This superaustenitic stainless does have its drawbacks, though. For one, the price for 904L metal can top the price of 316L by almost three times, making watches made out of 904L steel considerably more expensive. Additionally, 904L stainless requires specialized equipment to process it into watch cases and bracelets because it's much harder to form than 316L.
Is the added protection and polish of 904L worth the extra price? If you like the luxurious look of highly-polished stainless steel and the added security of knowing that your watch is virtually indestructible, then yes. However, if you feel a 316L stainless steel Grand Seiko or Omega looks just as sharp and you're ok with an almost-virtually indestructible watch, then save some money and stick with the industry standard.