My 2001 Cadillac Deville got head gasket leak, how can I put in the Head Gasket Leak Sealer in to radiator?

You don't. Head gasket sealer is designed to seal small voids in head gaskets using the heat of the cylinder head. The core plugs aren't hot enough to make the sealer work. You need a sealer like radiator sealer that works by plugging the leak with ground up wood pulp. Even then, it's only a temporary fix.

Are you sure its oil? If you bought it used someone probably added Bars Leak or some other cooling system sealer which can appear as oil. Cadillac Northstar engines are well known for cooling system problems, and that is usually the first step people take. If it is oil that could indicate an intake manifold leak or a head gasket leak, although it usually results in coolant in the oil, not the other way around.

I don't recommend pellets they will clog the radiator and heater core..if you have a radiator leak replace it....trying to do a cheap fix will only come back to bite you.if you put stop leak it will reduce you cooling,and cause it to blow a head gasket.that cost 10 times more than a 100.00radiator

you got slight head gasket leak engine blowby gases getting oil in cooling system have engine compression tested. need find problem first the radiator shop can flush clean out radiator

Computer glitch , have it checked for DTC'S - diagnostic trouble codes . Not all codes turn on the check engine light . Plus DTC'S for the BCM - body control module won't turn the check engine light on at all , could turn on a yellow wrench symbol on the insturment cluster .
Stop putting head gasket sealer into the cooling system , could clog it up.

You may be getting it from the oil or trans. cooler in the radiator. Get a donor radiator from pullapart.

The correct way is to remove the head(s) have the head(s) checked for warpage repair as necessary then clean everything good and replace the head gaskets. There are some places that advertise a product that is suppose to seal a head gasket leak but remember if it seals the headgasket leak it may stop up your radiator, also it will just be a temporary repair

MOST OF THE TIME WHEN CAR IS LEAKING ANTIFREEZE.CHECK ALL RADIATOR HOSES AND AROUND THE RADIATOR ON THE SIDES AND FRONT AREA WHERE COOLANT FAN IS.BUT BE CAREFUL DONT PUT HAND NO WHERE AROUND COOLING FAN.BECAUSE IT COULD COME ON WHILE YOUR HAND IS NEAR YOU CAN GET INJURED.ALSO CHECK AROUND WATER PUMP GASKET AND WEEP HOLE FOR LEAKING.IF YOU FIND NO LEAKS THEN YOUR HEAD GASKET LEAKING.CHECK ENGINE OIL IF OIL LOOK LIKE MILK SHAKE YOU HAVE HEAD GASKET LEAK.

A head gasket leak is usually indicated (among other things) by continous overheating of the motor, and subsequent loss of coolant.
It can also be indicated by the presence of engine combustion gas bubbles in the coolant resevoir.
Ensure the coolant system has no external leaks, such as faulty hose/s, radiator etc.

Ensure motor tune or other motor mechanical condition is not the cause of overheating.
I assume the motor is actually running hot.

Check cooling fans are operating ok (as necessary when car warms up):- if not check
1) fuse,
2) relay operation,
3) heat sensor switch.

Check/replace the thermostat,
Check the coolant pressure cap is serviceable & of correct rating,
Ensure cooling system is correctly filled, bleed off any airlocks as necessary when re-filling,
Ensure radiator hose on suction side of waterpump is not collapsing at engine revs above idle (due softening),
If the motor has been severely overheated, the head gasket could be blown - look for combustion gas bubbles in the coolant resevoir.

do me a favor and pull the sparkplugs out get a compression gadge and do compression tests on all the cylinders they should be within ten psi of each other if not yes you have either a cracked head or a blown head gasket