I'm reading conflicting reports for gl4 and gl5 and vice versa.
The reason 091 cores are so expensive is because most of them started out using GL-4, and a lot of them never survived Bus life because of it.
You absolutely need the increased extreme pressure rating of a GL-5 oil.
For a while, Rancho was recommending this oil:
http://www.redlineoil.com/product.aspx?pid=44&pcid=8