Grade 3, 18 Wheeler, et al...
I am going to speculate here that aircraft certified under CAR 3 regulations (which might possibly include the Metro 3 and the PA34-200) don't have a procedure for low oil pressure because the CAR 3 cert regulations did not require such a procedure be provided, and that aircraft certified under SFAR 23, or FAR 23 at amendment 7 and later, do have such a procedure, simply because the cert regulations require it.
I have not done any research on this, so, this is pure speculation at this point, but I kind of suspect this might be the case. What got me thinking about this is the model names associated with the Merlin - the numbers '3' and '23' seem to be more than coincidental, as SFAR 23 (a rule that applied to a rather small number of 10 to 19 seaters built in the late 1960s and early 1970s) was the legislation that bridged the gap between CAR 3 cert regulations and the present-day FAR 23.
Michael