I've just replaced the 18SE engine in my Mk2 Astra SXi with another one (the old engine snapped a rod) from a breakers yard.
New engine went in great, although we couldn't get the oil cooler to plumb in so removed it and all the associated gubbins.
Anyhow - got a wee problem. Engine runs fine but if you drive it for more than 5 mins the low oil light comes on and, when you check the dipstick it's well under the minimum mark. I've found that if you then leave the car for a number of hours and check the dipstick again it shows full, but start her up and within 5 minutes we're back in the oil light/very low oil level situation. Also, if you add more oil when it's low it makes no immediate difference to the level, but, again, leave it for a few hrs and check the dipstick and the oil level will be higher than before you started so the added oil has mixed in.
I've concluded that the oil is being pumped up to the top of the engine but, for some reason is taking absolutely ages to return to the sump to be reused. However, i know the correct amount of oil was put in originally and I can't see oil sloshing around in the cam/valve train area when it's showing low on the dipstick so a) where the hell's it going and b) what's stopping it returning to the sump!