Yeah but any good engineer will tell ya when ya hump an dump an generally maul an engine or gearbox you release all sorts of crap that was stuck in allsorts of places into said component (you can't tell I used to work in an early failure detection unit can ya lol) and that standard practice post engine/transmission change is to drain and flush and replace All filters (we used to then send them for analysis including an oil sample lookin at PPM parts per million of metals suspended in the oil) So for the sake of £30 I will whack 2 cans of fully sythentic oil through the blighter over the tuning runs. Bear in mind post chinook helicopter aft transmision change we run it for approx 20 minutes on the ground and empty the oil out and refill at 125 litres of £2 a litre oil then run it for 20 more minutes and do it again, Oh and again between 5 and 25 flying hours.