not sure what the mixture would matter in this case, but you are correct, need to check the oil for brownish watery film, it's either burning it, leaking it internaly via head gasket or externally somewhere, the cyl. with the 10psi diff. stands out to me, it could mean it's leaking there and causing high comp on that cyl. the same as it would when doing a wet comp. check, which would increase psi on that cyl.