[ODE] Combining contact surface parameters

[Alex Rubinsteyn]

I know this has come up on this list before, but I couldn't find any sort of 
definitive answer. 

When colliding two geoms with different contact surfaces parameters, what is 
the most realistic (in appearance) method of combining their surfaces? 

The suggestions I've seen were:

1) having a lookup table for all combinations of surfaces
2) averaging values (1st mu = 0.1, 2nd mu = 0.9, collision mu = 0.5)
3) multiplying values ( 1st mu = 0.1, 2nd mu = 0.9, collision mu = 0.09)

The first is impossible for me, since I won't know what all the possible surfaces 
will be in advance. 
So between averaging and multiplying, what gives better results? 
Is there some other method? (taking the max/min for some parameters?)

Thanks, 
Alex