According to the Rover manual the following is specified for diesel 825 models up to 1994, (and may be the same for later models - if in doubt then check). 15W/40 engine oil specification RES.22.OL.PD2 or CCMC PD2 (Such as Castrol GTD 15w40) 

Oil Filter type: I believe that Champion C143 was the original fitment, these seem to have become unavailable recently.  I have used instead Mahle OC 67 and lately Mann W940/1 easily available in my locality and a quality product. 

About 6.4 litres are actually needed for an oil and filter change so remember to buy the extra as you usually get the oil in a 5 litre pack.

The oil drain plug is situated on the rear right hand lower part of the sump housing. It needs a 8mm Allen key to undo it. You can just make it out in the photo. The plug has a copper washer. I also turn the steering to full left lock, this makes it a bit easier to reach the drain plug.

I found a plastic tray at the local DIY shop in the gardening department which easily holds the old oil, it's worth putting an old sheet of plywood or hardboard underneath, it will help soak up any spilled oil.

Once the old oil is drained the old filter is removed. The filter is situated at the rear right side of the engine and is accessible from above. There is just enough room for a filter wrench to undo it. After smearing a little fresh oil on the new filter's sealing ring it is screwed on. I do it up hand tight and then just put another one eight of a turn on with the wrench. You should note that the pipes in this area can be very hot.

Just goes to prove "The Mirror" can be quite absorbing. Funnels (tun-dish) for pouring in the new oil and for emptying the old oil, can be made by cutting off the top of plastic drinks bottles.

I pour the drained oil into the now empty container for disposal, (yes 6 litres or so always seems to fit in the 5 litre container!) Old engine oil is nasty stuff, dispose of it and the old filter responsibly.