Personally I wouldn’t unless the oil has been in for more than 10k or so miles. Better to change every 3k miles. Fresh oil & filter, don’t touch the strainer the bolt has a tendency to break the case if overtightened. Plenty of what oil threads on here but the favourite is Millers pistoneeze classic 15W40
Miller oily stuff already ordered (Demon Tweaks with a £5 discount). Miller also do engine flush - hence the question. Will avoid flushing the engine then
Really it's not important if you are changing oil regularly anyway. Just another bottle that sells for £££ with ingredients costing pennies and an advertising budget paid from ££ profits. I am sure somebody still has a bottle of the Teflon oil additive stuff that "let you run out of oil totally on a water cooled car and drive for a thousand miles." Sometime back in the 70s AFAIR.
Slick 50? ‘The original Slick50 PTFE treatment has been updated with Cerflon® technology to create a ceramic reinforcement of your internal engine parts.’
