I have some experiences with Mobil-1 15w-50.  I have been using it for three
years in my 82 rabbit truck with 175K miles.  I have put on 47K miles using
Mobil-1.  The oil temps are about 220F on normal driving.  It will reach 275F
on high speed driving (65-75 mph) but I don't have to worry about regular oil
breaking down.  My mileage is still the same before switching (30-33mpg).
The oil pressures are: 100# idle/cold, 25# idle/hot, 100# running/cold and
75# running/hot.  I did replace the valve cover and oil pan gaskets late last
year.  They were leaking anyway but the new ones have not leaked since then
(about 10K miles ago).  The inside of the head is very clean.

I also use Mobil-1 in my 83 GTI with 91K miles.  The mileage, temps, and
pressures are the same since changing over.  It does not leak (yet).  My first
concern was four years ago after I had just bought the car.  I found I had an
overheating problem at high speeds.  The water and oil temps were very high.
At one time I saw the oil temp go to 345F!  I have sinced changed the entire
cooling system and now don't have any overheating problems.  BTW, both vehicles
do not have oil coolers.  YMMV.

An interesting side note about synthetic oils.  I get Mobil-1 at Kmart or
Walmart.  But most of the time they are sold out but still have plenty of
other brands of synthetics.  Of course, it's been around longer but I guess
people really like the stuff better.

Jonathan
jdove@gsvms2.cc.gasou.edu