I use it in my AA1 and have had no problems. The engine seemed to drink quite a bit of it just after the annual, but has settled down again now.
Here's the blurb from the Exxon elite web site as to why its only semi synthetic.
Exxon Elite is a blend of synthetic and mineral-based oils (plus a highly effective additive package). Why a semi-synthetic? Our engineers determined that a fully synthetic oil may not have the solvency to handle the lead deposits that result from the use of leaded fuel. So they developed Exxon Elite as a semi-synthetic formulation that combines the best of both synthetics and conventional oils.
As a multigrade oil, it offers the flexibility to lubricate effectively over a wider range of temperatures than monograde oils. Compared to a monograde oil, a multigrade oil provides better cold-start protection and a stronger lubricant film at typical operating temperatures. Other benefits include lower oil consumption and better fuel economy. As a premium multigrade oil, Exxon Elite has all of these intrinsic advantages.