Garak,
You make a fair point, and an interesting observation about taxis. It is normal for taxis to get very high mileages. Why so high when most peoples cars manage on average 350,000? because the taxis usually are left running for most of their work day. Whereas a normally used car will be started and stopped numerous times a day.
Most damage to engines occurs at start up, almost everyone would agree to this. Garak, you have been using oil additives all your life, they are already in the oil. But the additive percentages are designed to satisfy all automotive applications, one oil "formula" to suit every car, this is different from viscosity. Additive only manufacturers are servicing the market that needs a specific need fulfilled. Chefs don't buy their knives from the supermarket, they see a specialist store. The oil you buy, with it's "perfect" formula of additives already added, is intended to suit any car, in general, so do you think the additives that are in your oil will offer the best protection for your needs? Maybe you 4Wheel drive all weekend? And yet you use the same oil as "Karen" who drops the kids off to school in the morning and collects them in the arvo in her 95' Honda Accord.
Now, the term "stabilser", while being a bloody odd name for an additive, may actually mean something. While I am not an expert on oils, I will bluster along. "Sheer" is something you do not want to occur to your oil, something like the tiny film that protects the metal surfaces gets thinner and thinner as the oil breaks down. From what little I know, gearbox or diff oil has a very high sheer factor in the oil. From what I know, "stabilsers" supposedly contain high levels of additives to prevent oil sheer. So, the term "stabilser" may mean "prevent continued decay or breakdown of oil". Or like in a hospital ward, "his condition has stabilsed, not improved, nor worsened"
Now, if you change your oil every 5000 miles or kms, regardless of what oil, you most likely do not need additional additives, as your oil is just starting to break down. If you are lazy and want to change less, use synthetic. Anyway, for those whose lubricant needs exceed what the big companies offer, use additives! I use them because my bikes are very high revving, quite old, and air cooled. So, engineering tolerances are alot looser than modern water cooled bike engines, hence the need for an extra thick layer of oil film between my metal bits.