crush washers

I agree that it does seem pretty random, but, if you look up the properties, you'll see that annealed Cu materials have more elongation at break (ductility), 40~60%, versus the full-soft Aluminum materials, 20~30%, at the same yield strengths (say 10~14 ksi). The most ductlie Aluminum material is maybe ~40% elongation max, but the yield strength is way down (~4 ksi). This implies that the "sealability" of the Cu (the ability to absorb the irregularities) is better at the same strength. On the Honda and Subaru that I've owned, I've noted that they use Aluminum washers everywhere except for the fuel line and brake line connections. On the F, I can't recall any Alum washers being used, but if the sealing surfaces are parallel and in good shape I doubt that you'd ever have a problem on a low-pressure connection with the Alum (but I wouldn't substitute Alum for the Cu on a high-pressure fuel connection or the brakes).

In some very specialized uses, you might need the better electrical and thermal conductivity of the Cu material. The Cu also has somewhat better general corrosion resistance, but the particular galvonic situation might also be a reason to pick one over the other.

Do you have some Alum washers in use on your F?

 

Or maybe they use aluminum on areas where it doesn't create an electrolysis problem.
Is your block Al?