today i noticed that on my tire i had crack marks where it came in contact with the road on the sidewall.

how serious is that?

by the way this is the only tire that has a slow leak.

 

As you already know or you wouldn't have asked, it's time to replace them.
Tires have AT MOST a 5 year life span, but if they start to dry out and crack you are running the risk of a blow out at high speeds. There is a date on your side wall to tell you how old the tire is.

 

Serious enough they should be replaced. Means the oil has dried out of the tire and it's dry rotting. Running on borrowed time.

 

Michelin provided test to US government showing extreme durability problems after 20,000 miles [lesser and lower quality tires] not they will fail just the likelyhood increases exponentially.

Why do you think oem spec fast wearing tires [besides wet braking and handling tests for magazines] because they want to make sure you have to replace tires while warranty is still in force.

 

yeah they are about four years old with 19,000 miles on them. orgininal tires.

il put some new ones on.

oem or any suggestions?

 

I would suggest some Nitto Invo I loved mine.