The following paper introduces a retrofit proposal for marine fuel oil system arranged to burn heavy fuel oil and low-sulphur distillate grade. In the face of tighter requirements regarding sulphur content in fuel, which are obligatory both inside Emission Control Areas and among international waters, it is necessary to provide safe and efficient ship operation. Conventional methods for fuel change-over between heavy fuel oil and distillate grade are saddled with necessity of constant process control and compulsory engine load reduction during the realization of procedure. It ensures that maximum fuel temperature gradient amounts 2 °C/min, which protects injection system against thermal shock. The solution proposed by MAN Diesel & Turbo in cooperation with LEMAG Lehman & Michels turns mentioned procedure into unusual until now level thanks to guaranteed safety, control, smoothness and degree of automation. So-called Diesel Switch constitutes automatic fuel change-over system, which implements the process free from engine load factor. A built-in programmable logic controller and specially designed nonproportional valve enable to achieve smooth and linear characteristic of change. Furthermore, device controls other fuel oil system elements and also registers procedure run. The second element in retrofitted system concerns the way of distillate fuel cooling. Sulphur in the chemical bonds with other fuel components reveals lubricating properties. Reduction of its amount simultaneously with usage of low-viscosity distillate fuel creates a risk of damage to plunger and barrel assemblies. A guarantee of proper hydrodynamic lubrication film in injection system involves maintaining a minimal fuel viscosity of 2 cSt at engine inlet. Commonly used central cooling system on ships has a significant limitation. Water from low temperature cooling circuit, which provides a coolant cannot decrease fuel temperature below 40 °C. Applied solution proposed by Novenco is concerned with chilled water system assembly, which can cool down fuel even below 0 °C through vapour compression or an absorption cycle. As a result, low-sulphur fuel viscosity can be increased at any load independently of seawater temperature.