Most Walmarts (US anyway) have in stock Shell Rotella 15W-40 (4 quarts 3.8L) for around $14 USD and Mobil 1 Synthetic 15W-50 ~$26 USD (5 quarts/4.7L); both have 1200 or more of ZDDP IIRC.
Sorry, a bit late to this thread…Spares Company have the filters but min order is £20 I think.
All Mobil 1 oils are synthetic, but are not otherwise necessarily the same. In a post #43, Paul Coene pasted in a page from the Mobil Oil web site that lists a number of their oils and gives the ZDDP numbers.Mobil 1 synthetic has 1000 ppm zddp.
It did not do well in scar testing.
Mobil 1 vtwin oil did ok in testing. I'm not sure what the zddp level is in the Vtwin oil, couldn't find that info.
ZDDP levels for classic cars | Mobil
Mobil 1™ FS 0W-40 oil already contains a higher level of ZDDP (1,000 ppm) that could benefit your flat tappet engine.www.mobil.com