I accidentally put 1/2 quart of 10w30 in engine that takes 5w30,  will it damage the engine? ?

Mixing the viscosity of oils will not have any effect on the engine. 5w30 and 10w30 engine oils have close viscosity, and thus there is no harm in mixing them.

No, nothing will be damaged.

How did you discover your so-called accident.  Mixing 10w in any amount with 5w in any car will not hurt anything.  If you are that ignorant, you should not be doing anything under the hood.

Were the valve guides checked for correct clearance to the valve stems? .0015 - .002 is the correct clearance.

I've been running 10w 30w in my last 3 Pontiac's with the 3.8 liter motor in them. All the oil filler caps had 10w 30w on them. You'll be okay. Just check your oil level every 1,000 miles and top it up. I've been running Valvoline High Mileage blend and change it every 3,000 to 4,000 miles. I've gotten 285,000 miles on them all and they were still going strong when I sold them. Keep an eye on the brake lines. They all rust up and need replacing.

No. 

Accidentally?

When did you discover that it was an accident?? 

An 18 year old car has other problems that you should worry about.

Now you have 6W30 oil.

Ho hum...

No.                           

Crack the owners manual, it will tell you 10w30 is allowable above -10F.

OMG!!!! Now you have to trade the car in before the engine seizes

i would hope not