On-engine filters been changed?
Lift pump. They are cheap from a tractor dealer and do fail. Change it out with a new gasket as they are also the number one cause of oil leaks. Keep the old one as a spare.
Check that the bleed screws on the Simms pump have good copper washers. I chased an air leak for months and it turned out to be a tiny copper washer under a screw on the pump. It took a couple of hours of running to finally get enough air in the Simms to stop the engine.
Things I would do to a new-to-me Lehman:
Change the oil and filter
Change the Simms oil
Change the on-engine fuel filters
Change the Racors if you have them
Buy another lift pump and keep the old one for a spare
Unless otherwise documented, change the 2 heat exchangers and service and keep the oldies for spares - water in the oil = many boat dollars
Change the raw water impeller
Check the belts and hoses and make sure the raw water system has double clamps
Exhaust system good?
Sooner rather than later, take off the valve cover and check the injector lines for leakage, it's easy to replace the copper washers. If they leak they will dilute your oil. If there is no documented servicing of the injectors and it smokes too much and puts a lot of fuel in the water, bearing in mind they all smoke and put a lot of fuel in the water(!), service them.
Check/set the valves
Retorque the cylinder head bolts - I know the manual only says to do the turbo engine but....
ASAP get Bob Smith's coolant tank
Do the Simms mod (drill the filler cap if no hole there already, remove and block the drain pipe, never again remove, or permanently plug, the level hole; change the drain plug for a small valve). You will have to measure the Simms oil forever after (big deal).