Firstly, is the engine leaking oil?
Have you noticed excessive smoke, if so is it white and smelly.
If you loose oil the crankshaft oil seal could be leaking and also sucking air which can lead tithe death of a piston.
I could pay to leak test the engine.
Warm up the engine, drain all the old oil out and refill with 650 ml or either 10w30 engine oil with no friction modifiers or ATF type F.