I have just been having a look, and know the one you mean. Only picture I could find was from 2017, and was small.

Its funny, it was on the same page as their mowers and trimmers - wonder how many of the ones they sold in 2017 are still running

It would have to be JASO FD grade. FA&FB are generally minerals or very poor semi-synthetic, most semis are FC and full synthetic are FD. I don't know how they come to the conclusion of of the ratings, but FD means very little ash and carbon build up is allowed.

From the MSDS (bearing in mind I am no chemist), I can see the flashpoint of the greenskeeper mineral oil (when it will burn) is 115 degrees, whereas full synth is 94 degrees. semi is in the middle at 97 degrees.
That means a more complete and clean burn (or at least thats my understanding)