when took my car for oil change, mechanic said that the tire Threads are too thin to drive , less than 5. So it is not safe to drive. Till what depth we can drive the car safely. Tires also has some minor cracks.
According to TireRack.com:
In most parts of the world, tires are considered to be legally worn out when they reach 2/32" (approximately 1.6mm) of remaining tread depth. U.S. law requires tires to have easy-to-see Tread Wear Indicator bars running from one side of their tread design to the other when the tire's tread has worn down to the minimum legal limit of 2/32 inch.
Tread wear indicator bars on a partially worn tire:
... if snow-covered roads are a concern, it’s time to replace your tires when they reach approximately 6/32” of remaining tread depth. For rain and wet roads, you should consider replacing your tires when they reach approximately 4/32” of remaining tread depth.
While you can possibly get away with running tread that thin, it really isn't advised to do so. The place where you got your oil changed at is right in suggesting you get new tires, but they are still legal. My suggestion is to invest in some new tires soon, especially with the winter months fast approaching.