A lot of stuff being said about the Exxon muiti-grade but I have yet to see any real hard evidence of problems with this oil IF it is used in an engine that has fresh from overhaul (after the inital break in with S80/100).
The problems seem to start if you switch to a multi-grade after more than about 400 hours, the fact that these oils clean the carbon out of the system and some of it is deposited in the oilways seems to be at the core of the problem.
There seems to be very little said about the Shell multi-grade the product has been on the market for the best part of 15 years now and any problems with this oil should have become apparent by now.
I have now started running Shell multi-grade in three of my engines that have been recently overhauled and only time will tell if this is the right thing to do but the engine that I have direct operating control over has burnt less that 1 Ltr of oil in the last 26 flying hours and I suspect the other Two engines have burnt about the same (but I don't have the numbers to prove it !).
Corrosion is a big problem with low use engines and the Shell oil seems to combat this , Cessna 152 engine rocker boxes are a very good illistration of this as the rocker boxes sit in the "cold" air above the baffels and are subject to a lot of condensation at the moment it would seem that the Shell oil is controlling the corrosion in the rocker boxes much better than the "W" oils but the next 150hour/Annual check will be the real test of this as the problem is much more apparent in the winter.
The next big marker will be at around 11-1200 hours when I would expect the Cessna engines to need a top end overhaul if the engines run to say 1500 hours then the almost 100% increase in the oil costs will have been a good investment.