Generally the older air cooled Lister and Petter diesel engines prefer to run on non synthetic oils, as synthetic oils tend to cause bore glazing and hence poor starting from cold as you describe.
I had a similar experience years ago as a plant fitter with a lister ST2 fitted to a lighting tower. It quickly became a PITA to cold start after semi synthetic oil was mistakenly used during a routine service, and the cure was to change back to the correct grade of oil, and the engine was run hard and hot for a couple of hours to deglaze the cylinder bores.
Not all Petter PH1 engines have an oil filter, but if fitted it will be obvious.