Hi Phil
As JB says, if you're only doing 1k a year then if the filter is new there is no need to change it. If the car is "new to you" and this is your first oil change then I'd be tempted to fit a new one so you are sure of the service life. The original schedules were to change the oil every 3,000 miles and the filter at 6,000 miles, so a new filter is going to last you for 6 years !
The filter is nothing special, it's a common Ford part and I bought an equivalent from my local Halfords. Yes, that common
The sump plug washer on mine is copper, I've never used nylon ones so can't comment as to whether it is a better option or not. I'd guess OEM was copper.
As for oil, here's where I might need a flame suit.
I'm using Mobil 0/40 in the Elan & Europa. I've run with Mobil 1 since the 80s, initially their full synthetic 10/50 and when that became harder to get locally I moved to the more modern 0/40. The lower cold rating doesn't increase leakage and moving to 0/40 didn't show any difference in the pressure gauge readings, either at idle or above 2,000rpm. Cold start is supposedly when most wear occurs and I'd sooner get oil circulating easily, hence my preference for the lower cold rating.
I don't think I'd go to 0/30 on an old engine myself, but it's interesting to note that when the TC engine was fitted to the first Elans, the recommended oils were 10/30 grade in the S1 Elan manual. That changed to 20/50 later in the 60s of course but I did find the original reference intriguing especially as these days we're heading back to 30w oil for economy reasons (amongst others)
The ZDDP levels that JB refers to has been a controversial thing for a while as it has been reduced for modern engines. Back on my Mobil1 fanboy thing, Mobil are one of the few companies which publish their zinc levels alongside suggested applications.
https://mobiloil.com/~/media/amer/us/pvl/files/pdfs/mobil-1-oil-product-specs-guide.pdfBut in conclusion you can buy "classic" 20/50 from Halfords which will be fine and what the engine was using back in the 70s. Or you can use a modern synthetic and at 1k/year you'll be equally fine.
Brian