You can make it simple. Install the oil of your choice to the proper level and then drive the car. The proper level -- when you insert the rod and piston (what I called the dipstick") it should drop in with no resistance until the last half to 3/4 inch of engaging the threads. At that point you will feel the resistance of the oil. Now drive it until it misbehaves - spitting or stuttering on take off or when accelerating on the road. The mileage you covered since topping off the oil will be your interval. Realize--you are dealing with 100+ year old technology. It was still around when your car was new because it works--not because it's maintenance free. The fuel injected models that superseded the carbureted versions did so for greater performance and control for emissions. I love my 1969 144S with its twin SU carbs (similar in operation to the Stromberg). To replace my fuel system would require a mechanical fuel pump and 2 carburetor kits. Do do the dame for my 2002 S40 I'd need an expensive electric fuel pump, 4 new fuel injectors and I don't know how many electric relays and control units. - Dave