We have built (and run) some low volume molds for Ultem but we chose P-20 steel. Your 2° shut-off is tight for steel so I suppose aluminum would be even more subject to galling. We use electric instead of oil heat but the thermal expansion issues would be the same. Ultem requires very high injection pressure. I would be most concerned about the aluminum deflecting under pressure.
We have found that unless we are removing alot of steel (aluminum) from the mold blocks (i.e. deep cavity) then we are better off to just use steel. The only real savings you get from aluminum is machine time.