Hi!
I can't understand why this subject is comming up with regular intervalls??!
The oil percentage and type of oil and whether it's 15% or 20% is not that critical ........!
As long as you know what you do it does not matter if you run 15% syntetic oil or 25% castor oil when running in your engine ...both could be bad , or good ! Its how you set the fuelneedle that's counts!

Regards!
JanK
Sweden