You never said what the milage you are getting is or what type of driving you are doing or what type of fuel you are using. Is the car smoking at all.

If you do not have the $ right now concentrate on those thing that will not cost you anything like adjusting the valves and checking the timing on the fuel injection pump. Poor tire inflation uses up more fuel; things like that. I had a gas car and that started to use more fuel and found that the brake calipers were sticking enough to reduce the fuel milage but not enough do damage the brake pads.
I bought some new made in Germany spray nozzles from C Sean Watts at a resonable price. But you should always consider completely rebuilding the injectors with pop testing; if done correctly the 1st time and your fuel system is kept clean you are good for anothe 1K. That is the best economey in the long run.
To answer your other question; all of the injectors you are talking about have the same opeing pressure setting stamped on the side they can be interchanged. Non-turbos 115bar; 135bar for turbocharged. Years aprox.77-85.

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84 300D, 82 Volvo 244Gl Diesel