Our 1998 (S reg) Peugeot 306 1.4l petrol has a bit of a history for making tapping/knocking noises from the top part of the engine. I've always assumed it to be the tappets/hydraulic valve lifters. It seems worse at different times of the year (can't remember if worse in Winter or Summer) and usually is most noticeable at slow speeds in low gears, eg. when reversing off the drive.
It recently passed 113,000 miles and I did another oil + filter change (Castrol GTX, 10w40) and noticed it was a bit noisy again after changes. A couple of days later, whilst in Halfords, I saw some magic liquid (yes, I know....) which claimed to improve valve lifter noise. Simply pour into oil filler hole and it helps clean up the problems which causes noisy valve lifters. Claims to be able to be kept in the engine permanently (rather than X days and then do oil change). Thought I would give it a go, not sure if it would make any difference but thought it was unlikely to be a problem and at £5, wasn't really a problem if it didn't make it any better.
Needless to say since doing this the engine has been far far worse at tappet noise. I can just about notice it driving along at 30mph with the windows open. Reversing as before the noise is very very noticeable.
A couple of questions. What may be the reason for this noise happening in the first place? What might this 'snake oil' have done to make it worse noise (eg. made more or less viscous, thus losing oil between tappets, thus making a tapping noise).
Finally, would I be better off with a different grade of oil? Would something like 15W40 be better? I've seen that advertised for 'older engines'.
Thanks
David