Originally Posted by
deltahotel
Don't know which variant of the RB211 is fitted to the Monarch 757s, but the C engines don't specify a minimum oil quantity in flight, neither will you get a caution/warning at any specific level. So unless you spend your time watching the oil quantity on the lower EICAS screen, the first indication of oil loss is likely to be low oil pressure - a bit like your first car!
When Monarch had 757s, they were all -535E4 powered.