Vavle stem seals 750 commando

bEST  JIM SMITH VITON SEALS. a PART OF MANY OTHER INFO AND RACING STUFF

THE SEAL IS JUST AN FIT TO ORIGINAL INLET VALVE GUIDeS

jsmotorsport.com/product/viton-valve-guides-seals/

RGM also do Viton stem seals : https://www.rgmnorton.co.uk/buy/valve-stem-oil-seal_74.htm .

Having only ever used Norton ones, I can't comment on whether any of these are any longer lasting. I don't know what the original seal material was, or what Andover are selling today (although, reading between the lines of Roger's comment, they might be British Leyland parts). Viton is certainly preferable to the common butyl rubber but it's probably a bit hot around the valves for either of these to be especially durable. Teflon would be best but I don't know of any in that material.

Take a look at the JS Motorsport web site.