Many years ago detergent oil was not widely available and most just used plain old SAE 30, non-detergent. For those who switched to the new detergent oils that had already run many miles on the old non-detergent, engines began to blow up because the detergent oil statred to loosen up all the crud in the engine and clog oil passages starving bearing.

So the old adage was, never put detergent oil in an engine that had always run on non-detergent! And that still hold true today and you can still by non detergent oil.

Sounds like your mechanic is confused!