You could find that all the carbon/gum etc in the engine has been softened by the 'fresh' oil left in it. The debris will end up in the oil filter and block it (don't ask how I found out). Be prepared for two filter changes on the same oil.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i