Repair or replace

Hello! I have had this car for three years. I have put a lot of money into it, with the idea that I would keep it as long as it runs okay. And I thought my mechanic was on the same page.

My problem now is thin slick of oil (not transmission fluid) on top of coolant reservoir. Already did head gasket replacement and cylinder machining but that was two years ago.

My mechanic, who I do trust, says I should not pay more than $600.00 to have anything done, like to have the engine torn into to find out what is wrong. He says the car is not worth that much. While I appreciate his concern, I just put $1200.00 into the engine last fall at his shop, tracking down vacuum leak and other engine issues.

My question; if I do have an honest mechanic would I be stupid to put more money into the car? We have fixed a lot of things, but I know there are plenty of parts we have not repaired or replaced. It runs like a top even now. No sputtering, smooth shifting, it runs great and obviously I will keep it as long as it does.

Can you please let me know what you think? Thanks so much!

Colleen