Using Widman's calculator and the following viscosities:
Pennzoil Conventional 5w20: 53.3 @40C, 8.26 @100C (Taken from the VOA board.)
RT6 5w40: 87 @40C, 14.2 @100C (PDS info)
The final viscosities of a 50/50 mix are:
67.8 @ 40C
10.8 @ 100C
So it would be a 5w30, pretty much right in the middle of the grade.