I have used red line diesel engine cleaner in the 90 once after it was stored for 18 months - didnt really notice much diffence in performance but it did drop the emissions a bit for the mot ! - remember its the shape of a brick - with a roof rack !
It is better IMHO to make sure the timing is spot on, valves are adjusted correctly and new injectors will make the biggest difference in performance - also using decent oil and regular filter/fluid changes.
Though if it is running fine, not down on power and the servicing schedule is up to date I'd just leave it alone !
Cheers
Grif
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