The Lister S series is very well known for internal fuel leaks. This is one of the first things Lister technicians are trained in as far as routine maintenance and diagnosis for these models.
Since much of the fuel injection equipment is located inside the block - as you have observed, any of the lines and connections can be the culprit as well as pumps and injectors.
Here are some suggestions, in order of liklihood:
1.) leak-off line connections at the injectors. (low pressure)
2.) supply lines at the injection pumps, or dowty washers. (low pressure)
3.) delivery valve seals in the tops of the pumps. (high pressure)
4.) pump to injector line connections. (high pressure)
5.) diaphragm in the external fuel lift pump.
6.) injector caps, injection pump alignment screws, even faulty injector nozzles.
If your engine is running normally except for a rising oil level, you can open the valve covers and pump cover and run the engine while observing all the parts and connections. If the leak is very minor, and no source of contamination can be found, it may be necessary to dismantle and check each suspect part individually. If you see seepage or dripping fuel around a particular connection, sometimes simply tightening will do the trick. Pay attention to proper torque values, especially on the injection pumps as it is possible to damage parts from excessive tightening. The Dowty washers on the pumps should be replaced anytime you remove the lines, and pay attention to the belled flare connections on the high-pressure and leak-off lines as they can become damaged over time from over-torquing and just plain wear-and-tear. You will be able to see the deformations or ridges on these parts once damage has happened.
You can silver solder steel or braze all of the copper lines if any cracks are found, but not advisable to try repairing the high-pressure lines unless you have no other option available.
dieselgman