A steel washer would work fine on an aluminum pan as long as it was smooth and had no sharp surfaces to dig into the aluminum.
The crush washer is there to deform (crush) to any imperfections in the bolt head and / or bottom of the oil pan. Any moderately soft material will do although soft aluminum and copper are used most frequently. Aluminum is less expensive of course so that is why it is used almost everywhere as a crush washer other than on brake connections, which are almost always copper or soft brass.
Another thing that works well is a drain bolt with a groove for an O-ring in the face. The O-ring seals with minimal pressure and actually lasts a surprisingly long time but most people greatly over tighten them; the oil filler on the engine and final drive of the C-14 are good examples of both an O-ring sealed cap and a point where people greatly over tighten them.
Brian
Everybody seems to think the purpose of the aluminum washer is to provide an oil seal. I always thought it was to protect the aluminum pan from being damaged by the steel drain bolt.
