Well, it SURE ain't gonna' be a "15w-60"
5w +10w= 15w
-20 -40= -60

NO!!! You can probably mix it without overt problems, but OTOH, remember that the calculation of the resulting net viscosity is not a linear calculation. There's a logarithmic formula for calculating the true net vis resulting after mixing oils of different viscosities. It's here on BITOG somewhere - find the true formula, and make sure you're OK with the outcome before actually mixing oils.