What exactly was done to the engine? I'd guess it may have had new rings fitted in partly worn cylinders. Engine cylinders wear very slightly oval with the throw of the piston and rings wear to suit. However, if new rings are fitted in a worn cylinder they often won't bed in properly for obvious reasons. In such cases, oil will pass by and be blown out the exhaust or leak from exhaust manifold. IMO if there's any slight doubt of wear it's best to fit liners or bore out cylinders and fit over size pistons.
If your engine has indeed been fitted with new liners or re-bored then it could be the oil you have is too good. Modern oils are often so good they cause glazing and lubricate too much resulting in rings not bedding in. I've used a type of running in oil in the past where this poorer quality oil allows a limited amount of wear to initially take place and rings etc bed in. It's then changed at say 50 hours for quality oil and hopefully happy days !