It's a little bit boaring today on a holiday day (...), so when I recall right, idle is 650 with the automatic.
You will see, how far you can go before you BW box kicks you right in the when goeing in D or R.
I have my timing cover off at the moment, the oil pump gears are out. Honestly, without this forum I would have assembled it the wrong way. If you search over the web for the Rover V8 oil pressure, you will get the feeling, that millions of V8s died due to low oil pressure and I would not wonder, if the first tipp comes up to prime your oil pump with grease from whales, which must have an age of minimum 46 years.
It's not easy where to start, but both Harvey and Alex are right. From my side I would say, never use expensive oil (synth or half-synth), but p6club.com gives you the right oil quality advice. It's an engine from the 60s, so make frequent oil changes and do not use additives.
If you have to go deeper and you are finally ending taking off timing cover and/or oil pump unit, google for "rover v8 oil pressure". I found some helpful information on the richer-engineering.com site.
I.e. Using a new pressure relief valve (piston) can cause more problems than cure problems. If you do not have those informations, risk to do exactly the wrong way, is obvious.
Where I'm personally not clear, is the "updating" or improving of the P6 oil pump.
I have only found possibility of using a tadpole form valve, all other modifications require a different housing from the post P6 generation and those timing covers are hard to find.