Posting this as it might just help if you are in similar predicament with a sticky lifter. One of the lifters on my Elan has been refusing to work properly for some time (over 2 years) and has been getting steadily worse. Have tried different weight engine oils and some Liquid Moly lifter treatment - all with little or no improvement

Was just about to start working on physically removing the lifters when I found some positive comments on US forums about Marvel Mystery Oil (MMO) curing lifter issues. Managed to find some MMO online from a UK supplier for £13.
Anyway, warmed the engine a bit, drained about 350 ml of oil from the sump and topped up with 350 ml (10% of oil capacity) of the Mystery Oil. Have idled the engine with some short periods of revving the engine (2-3k) and within 1 hour of running the engine the lifter noise has gone!! I would say the engine has never sounded better at idle!!!!

Can't quite believe it yet as it has doggedly remained noisy for so long.
I would not feel happy driving the car with MMO in the engine, although the instructions say you can, as the oil pressure has dropped. So I will drain and change the oil before running on the road again. Fingers crossed that MMO has finally cleaned and cleared the lifter.
Hope this might help someone else with the same issue - Steve
https://www.ebay.co.uk/sch/i.html?_nkw=marvel+mystery+oil