Yes they can be changed in situ, but may not be the stem seals, oil gets thinner with heat, thinner oil is used quicker, what grade oil are you using ?, if it has a few miles on it I'd be using a 60 weight like mobil 1 10W60 from matt lewis for example, or if not many miles a 50 weight would be ok, anything thinner and it drinks the stuff if on std(old) engine parts.
In the last 400 miles on my engine(new build) it has not used a drop, but I have a step gap seal in the turbo, as this is where a lot of the oil goes.
tabetha