i think somebody is playing a trick on you converted,what makes you think there is detergent in optimax?mabye the spark plugs "carbonise" due to the oil your engine is consuming.

for the record bp ultimate is the best,cleanest and most consistent 98 octane on the market.
more people use it straight up or with additive in street racing circles than any other.

standard unleaded should be avoided at all costs.it barely meets australian standards for octane and quality.it causes damage to fuel injectors and clogs them up.
if you want to see what is in unleaded,get one litre in a clear container and leave it open in the sun untill it has evaporated,and see what is left in the container.