I bow to your superior knowledge. After checking, SI-1070 only specifically names Aeroshell 15-50 but states "other equivalent finished products" - presumably containing the necessary additive - are acceptable. Both the plus have the additive yet the Shell site doesn't mention conformity with 1070 along with the other approvals it shows.
I think when I first looked at this and started using UL91 it was SI-1070R not the current 1070S and it was when the future looked rosier because unleaded was considerably cheaper. Now my UL91 stickers next to the filler caps are fading and I doubt I will renew them since regular Avgas is now cheaper
