The manual recommends every 200hours or 4 months (whichever comes first).... However the 4236 was used in everything from London Buses, through tractors and lifting machines. Although my boat (new to me) came with both the basic manual and the Marine
supplement (the latter is important) on one occassion while away from the boat I needed some information and downloaded it in PDF form from I believe Perkins website( Can't remember). If you experience any difficulty, PM me I shall email
you the basic manual in PDF form. However you will need the marine
supplement and I seem to remember I couldn't find that.
In any event, personally I use the three month -100 hours regime, which I believe is the most common in the sailing community. ( habit) although the marine supplement also states 200 hours-4 months.
Finally its a very good and powerful engine...