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D4 would be the closest current model. You can get the hp from a D3, but they are a problem engine even at 190hp, let alone at 200hp+. A lot depends on why you are looking at changing. A proper rebuild on the 41 will probably last a long time and at a fraction of the cost of a new D4. D4 will be a little more fuel efficient and less smoky, but they are not cheap.
Sounds a bit strange. Is it at the back of the engine? Could it be from the rear oil seal on the crank? Oil gets hotter and under more pressure at higher rpm's, so where else can it come from that isn't visible? Another thought, check the sealing at the base of your dipstick tube. We fixed an oil leak on a KAD32 that took ages to find and found that the dip stick tube wasn't sealing to the block properly.
Of course I could be wrong about your engine! Attached is a picture that is similar to my engine, the oil breather filter is the silver cylinder near the air filter. Of course your engine could be different.
The relief valves only operate if the crankcase pressure gets too high, otherwise you would have no reason to be aware of them, they are not a service item.
Good luck.View attachment 9797
Sounds a bit strange. Is it at the back of the engine? Could it be from the rear oil seal on the crank? Oil gets hotter and under more pressure at higher rpm's, so where else can it come from that isn't visible? Another thought, check the sealing at the base of your dipstick tube. We fixed an oil leak on a KAD32 that took ages to find and found that the dip stick tube wasn't sealing to the block properly.