Hello,
That's correct, the Energy Saving Ordinance (Energieeinsparverordung) stipulates that heating boilers that are 30 years old must no longer be operated. This is your chance to sustainably reduce your energy costs! Since natural gas is available, I would definitely consider this option, as the investment costs are very low, and the benefits in terms of savings as well as other factors that are difficult to quantify are significant. By other factors, I mean that the gas condensing boiler will be much smaller than the existing boiler and you will also gain additional space by eliminating the oil tank. Investment costs just for the system are around €5,000 plus additional work and the dismantling of the old system including the tank. The remaining oil may not necessarily need to be used up or disposed of; some heating engineers purchase it. The measure is supported by the KfW development bank with a 10% grant or a low-interest loan. Is there already a gas connection in the house or does a natural gas line pass by the house, but a connection has not yet been made? Please check this and contact the local energy supplier. If there is no connection yet, check whether the local supplier supports your natural gas connection. Otherwise, costs of up to around €2,500 may arise, which can also be subsidized.
Overall efficiency 160 cannot be answered offhand, as there is too little information about the certificate. Consulting a qualified energy advisor can certainly uncover additional savings potential, which will definitely be worthwhile in the long term.
Best regards, Erik