Certainly JC!
I would reccomend Duckhams Classic 20/50. This oil has served me well for years.
On the oil filter front, make sure that you do get a filter for a P6 V8. A lot of motor factors these days will try and sell you one for a SD1, but that will not fit, as the threads are different.
Can't remember how much oil the V8 holds, but you will need to buy 2 full cans of the stuff - 1 ain't enough!
A quick tip- run the engine up nice and hot, and then drain the engine oil. But remember to wear gloves,and don't scald yourself! Before changing the oil filter, replace the drain plug, and fill the engine with oil first. I normally half fill the new oil filter with oil before hand, and then change the oil filter last. The object of the excercise is to give the oil pump as little chance as possible to drain. If they do, they can be a swine to get to pull oil up again, and the worse case scenario is having to strip the oil pump, and pack the gears with grease! I've never had this problem though.
Some people like to manually prime the oil pump, after changing the filter, by removing the distributor, and turning the oil pump driveshaft with a special tool. I've never done this though.