I seriously wouldn't risk it and would get it drained and replaced.
The 156 2.0 JTS unit is very sensitive to Engine Oil. I had one and I had to keep on top of the oil levels even with the correct spec (Selenia 10W-60 or Castrol Edge Sport 10W-60 Edge).
Mine burned a lot of oil (fairly common with the JTS) hence the reason for the thicker, 10w-60 grade.
Heaven knows how quick she'll burn through a thinner spec which consequently can leave your engine starved of oil, seizure and a costly rebuild.
If the mechanic got it wrong, he should be draining and refilling with the correct spec at no extra charge to you.