MIL-L-46000 B is the Military Specification (MilSpec) that the oil was designed for. It tells you what purpose the oil was made for, it's chemical and physical properties may also be included. Just about any lubricating oil can work in a MG-42, but I tend to stay with a lighter oil that does not have large amounts of paraffin or other waxes. Do not use greases of any kind. Greases are designed to release lubricating oils only under high pressure, sometimes under high pressure and high temperature, and they are usually too thick and may cause friction that may cause stoppages, misfeeds, poor extraction, and other problems. The MG-42 is so simple that it takes a lot to cause these problems. Greases also trap dirt, soot, and other deposits which will act as an abrasive on sliding surfaces (THIS IS BAD). Oil allows grits and dirt to move out and away from between sliding parts. You could even use motor oil on an MG-42 with no bad results, but the better the oil is, the easier it runs and the easier cleanup is.
Some may disagree with me, but I’ve found a super gun oil. I’ve run both turbo and supercharged engines and used an oil in them that does not break down in heat, does not coke and is super slick and doesn’t gum after it’s sits for years. Many of the top, most expensive sports cars call for its use right on their valve covers…Mobil 1 synthetic. I’ve used it in my Wise-lite M53 right from the start and whether it matters or not I don’t know, but she’s ran without a hiccup so far. That and I’ve used it in everything from Cetmes, G3’s and my own AK builds. For 5.00 a quart (lifetime gun supply) you can’t beat it.