Oil burning

Penrite hpr30 has a good reputation amongst essex v6 fans so I dont think it could be anything to do with the oil.
I take it that you havnt owned the car that long (1st oil change).
One possibility is that the engine was already tired in the hands of the previous owner so it was decided to run thicker oil. Going back to the thinner oil has caused more throw off from the big ends which of course lands on the bores and overloads the rings.
Another possibility is that the PCV valve is kaput which can happen anytime.

I'm using Castrol 20W/50 in the V6 Capri....and have used Castrol in all my cars for 30+ years.

Yes a compression test will show if its a localized problem (one cylinder is low) or a general tiredness (all cyls low and different). In any case there is probably plenty of gunk and carbon in there it may be time for a refresh and the best way to do it is remove the engine and put it to an engine stand to work on. Problem with working ensitu is the crossmember prevents from getting the sump off.

GTV6,
The car has been sat in the shed and only started to keep things sweet so to speak for the last 12 mths, because of a bit of a rebuild was required and also I have done a reno at home meaning the car could not be moved. Thinking about it the stem seals could have dried out in this time, or an oil ring could have become stuck. At this point I am kinda hopeing that I have not glazed the bores up, as the engine has been run a few times under no load. I kinda hope if it is a glaze issue that a couple long runs under load might get the rings to fully expand (now under 100% load) and brake the glaze, or that a stem seal change will be all that is required. Your thoughts on all this?
 
Capri

Yes it is quite possible that all it needs is some regular use.
I had a corolla  with the 4age engine sitting for about 4 years with starting it every month and recently it went to running on 3 cyl. Cleaned the plugs and checked all connnections but still running on 3. I got sick of it and gave it a good rev and suddenly its back to 4 and running like a swiss watch.  (sticky valve).
Warm it up and then give it a hard time, if it doesnt come good then at least you tried.