Regular Gas used where Premium required-Mercedes 2002 E320 (187K miles)

Chaud

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#1
I have been using regular gas in my car for 3 years when the car manual says Premium gas only. I made two assumptions before I switched to Regular
1. Car electronic system will notice this and automatically adjust to low Octane (87) gas.
2. Being switched to low octane I may get 3 to 5 miles per gallon.
Recently I read this type of thinking is wrong. Because regular gas will build carbon inside the engine and on the valves which will ruin the performance of the engine. Now I am worried if this is true or not. If so can I use some additive in gas or in oil to remove this carbon deposits or it is too late. The car is running fine but performance has suffered a bit and makes some lifters type of noise. Thank you for your opinion and suggestion.
Chaud
 
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#2
your fine - likely a very small performance issue- if you worry about CARBON build up - add 1 can of "seafoam" to gas - there is also TECHRON by chevron another brand that I would suggestt...
the lifter type noise ? unlikely fuel and as the the CEL is OFF and no codes... may try to isolate the noise top bottom front rear of engine and post again