As for the oil separator box, good thought and yes, the seal will almost certainly need replacement if you remove the box after all these years. These seals can be dirt cheap from on-line suppliers, but the shipping will kill you. I thought I'd mention when looking in local parts houses the 240/740/940 oil separator box seal is Volvo p/n 969330 and is the same as the 740/940 rear mounted distributor shaft seal (same or similar p/n). A fat regular rubber o-ring likely won't be up the task what with all that oil and heat.

Often times you'll see it recommened to replace the oil separator box rather than try to clean it when they start to become clogged with caked carbon. On that basis, I had a new one standing by as I figured the original would likely not be salvagable or would be brittle and break. I did not at all need it, but a shop would likely put in a new one just to save time. With enough poking around with screwdrivers and a stiff wire (actually my preferred spring steel strip salvaged out of old wiper blades), in amongst repeated flushings in a paint thinner bath and I was able to get it satisfactorily clean.

Getting the oil separator box out can be a bit tedious and hard on the back with all the bending over. The job can become quite a bit more difficult if there is a frozen EGR valve blocking access as is the case with some of the later B230s, possible yours. Those can be a real bear to get loose often requiring multiple goes with penetrating oil and suitably large counterheld wrenches. Think about using a large Crescent-type wrench or carefully using a plumbers pipe wrench if you get desparate as they often have a slightly shorter handles than a large combination wrench. As with many counterhold situations, having the handles just off from parallel and squeezing them together often gives you better leverage in awkward locations such as the EGR fittings. Once you get the rear fitting open, you can disconnect the other end up at the top end of the tube attached to the intake manifold rather than trying to separate it at the EGR valve. Don't forget to replace the connector wire on the EGR valve like I did. The Check Engine light came on some time later during driving when the engine finally got warm enough for the ECU to go looking for the EGR sensor -looking up the DTC code immediately told me what I'd missed, two seconds to plug it in and I was good to go after clearing the code.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now