Thanks Guys for your warm welcomes and all your responses -very interesting indeed (and very useful). Interesting to hear your comments on the Penrite brand, which is available here in the UK - shall be avoiding that with the points raised! I am researching oil here, armed with quite a bit of info I didn't know, so thanks.
I will drop the sump in the spring when it warms up a little here, as suggested by C Carl, as I don't know the history of the car since it's been in the UK, other than the previous owner, registered it here in 2012 and has done less than 250m miles since! I have never heard of putting a magnet into the sump (oil pan) before. It makes really good sense and someting I will look at. I assume you just find a good quality magnet and pop it into the oil pan and re-fit, having given it a good clean out first?
I have found Bob's Automobila store online, which seems to stock a good amount of Buick pre-war parts and it lists the sump gasket at $50 which seems pretty expensive, but I suppose that's the going rate. I also need to order a Shop manual for her, as the Reference book she came with doesn't give you much technical info.
If anyone can reccomend any other good dealers who stock parts for older Buicks, I would be most grateful as my previous car, a 1932 Austin 10/4, was really easy to find parts for as she was British made and supported by a very active club here. As previously mentioned older American cars are pretty rare in the UK and spares etc virtually non existant, so I am sure I will have a few technical and spares related questions going forward.
Pretty foul weather here this weekend, so lit the stove in the workshop and got on with a few jobs on her -roll on the spring, when I can get some miles on her...