Not all LSD's require an additive, only those that involve friction based designs such as clutches. These are the most common designs but there are others that for example just use gears (eg Truetrac). The additive is a friction modifier that gives the correct degree of slip and therefore prevents shuddering etc. The safest bet is to probably go with the OEM's recommendation but that's not always possible. If it isn't, then a 75W140 LSD
oil is probably the safest bet followed by an 80W140. A 75W140 will definitely be
synthetic in nature and most likely the 80W140 will be as well. There is no downside to using a
synthetic (other than cost), only upside from a performance perspective. The treat rate of the additive (depending on the actual additive) is generally in the range of 3-6%. For example in the Holden/Ford V8 diffs, it was common to have to use 6% of the Sturaco 7098 so some manufactures (eg Nulon) now add 6% to the
oil you can buy so no more additive is required. Many diff builders still swear by just using the LS90 mineral
oil in everything, but I don't agree.