My dad and I have been using Mobil 1 for about 30 years and have always had very good results from it. In my recent hunt for Birfield grease choices, I discovered that in 1999 a law was passed that created a loophole allowing oil manufacturers to label regular petroleum oil as "synthetic". Basically, TRUE synthetics use Group IV or Group V base oil stocks (non petroleum). In 1999, a loophole was created to allow lubricants made with Group III (petroleum) base oil stocks to be classified as "synthetic" if the sulphur was removed from the oil.
Although regular petroleum oil has come a LONG way in the last 30 years, we still felt rather deceived by this labeling loophole. Mobil 1 does not make it very clear anymore (without a lot of online digging) what their base stock is, they used to clearly state that their oil was Group IV.
Thus, I have switched to Amsoil which still offers SOME true synthetic oil choices (they also offer some products that use Group III base stock). Amsoil seems to be much more open about their products and which base stock is used in each product.
I believe it is very likely that Mobil 1 may be using Group III base stock for many of its other "synthetic" lubricants, grease, gear oil, transmission fluid etc...
I was a huge fan of Mobil 1 up until now, ExxonMobil is now on my **** list

. I guess I just don't like being told a product is "synthetic" if its made from the same stuff as all the conventional oil.
Interestingly enough, Group III base stocks are considered synthetic motor oil ONLY in the United States; elsewhere they are not allowed to be marketed as "synthetic".