My mechanic is a friend, not a person who would rip me off. My engine light came on.
It happened a few months ago too and always after major traffic jam into the city.

He used his diagnostic device and said he found a small leak in the engine. He said
I can take it to the dealer (oh, I have a 2002 Hyunday Elantra) but it will probably
Cost me a good deal of $$. He suggested I continue to use the car. He put out the
Light. What is your opinion.

We need to know what is leaking to start with before any of us could even guess for you.

I'm not a mechanic who would rip you off either. I like to help car distress victims out. For free!
However, my opinion is going to be a complete shot in the dark because we need as much information as possible to help us to help you.
For instance, your mechanic friend must have given you a number, (a code) as to why your engine light was on.
Let's see... extended idling in heavy traffic jams. Possible lazy 02 sensors? Bad catalyst?
Oil leaks... hmmm, Hyunday... okay, another shot in the dark here but I'll say the timing cover because I've just seen one done in the shop, and yes, that looked like a pricey repair.
But I can't tell you to just keep on driving it from this end. Hope I helped.