I have an old Homelite ST 200 string Trimmer model UT 20501-B.
I have been searching a long time and can't find the proper Oil/Gas mixture,
Nor can I find an owners manual that will tell me which spark plug is the right one.
Help is greatly appreciated.

:):)

Hi,
Mix is now 50:1 with an appropriate oil. Plug is DJ-8J in Champion. And manual (believe it or not) is:
http://www.odref.com/homelite/string-trimmers/ut-20522-522a.PDF
Peace,
Clarke