This will make some of you laugh: I bought two pre-slimed tubes yestarday and tried to fit them... the first one, I managed to nick somehow. Thought I had been careless - I have never nicked an inner tube while fitting them before. The hole sort-of sealed, but the tyre would not maintain pressure. So I tried again with the second tube - and despite being extremely careful, somehow managed to nick that one too. This time, it didn't seal at all. Deflated, both me and two tubes, at a total cost of £19.98 for two tubes. The rubber seemed very delicate, compared to my normal Impac. The manufacturuing dates on the bottom of the pack indicate 2019, which could have caused the rubber to soften maybe? The life of slime tubes is two years. I don't think I'll be using slime inner tubes, because it also seems that slime can damage rim metal:
www.slime.com
I'm just going to put on Marathon Plus tyres, and carry a spare Impact inner tube, after I've thoroughly washed my wheels of slime...