What should I replace when I fit a used N63B44T3 engine?

A used engine lasts longest when it starts life in your car with fresh fluids and new seals. Clean the intercooler and the intake pipes, and fill the turbo oil feed with clean oil before the first start, so the turbo never runs dry. Fit a new oil filter and fresh oil of the grade BMW lists for this engine, and refill the coolant with the correct BMW antifreeze mix. Use new gaskets for the exhaust manifold, the intake and the sump plug, and check every hose clip before the first start.

Look at the engine mounts and the clutch or the torque converter while the engine is out. Old mounts can crack a new engine's brackets, and a worn clutch is much easier to change now.

Before the first start, turn the engine over without fuel until the oil pressure light goes out. Then let it idle, check for leaks, and drive gently for the first few hundred kilometres.

The warranty asks for fitting by a qualified mechanic and the correct oil, coolant and filters. Keep your receipts in case you ever need to make a claim.