Depending on the model of the Craftsman lawnmower, there is either an oil plug underneath the deck of the mower which needs removed to drain the oil or the lawnmower needs tipped on it's side to allow the oil to drain out through the oil cap area. Once oil has drained you will want to refill the oil until the dipstick registers "full".
youtube - push mower oil change.... oil plug under mower deck
30 wt
Click the link and then look up your engine based on the model number.
what grade of oil on this engine
Your owner's manual should explain that.
5w30
30 weight oil can use car oil or lawnmower oil same oil
The oil drain plug for the Craftsman 625 series lawnmower is located on the bottom of the mower, just above the blade. Tip the mower back. The plug is round and threaded with a square head. Using the square head of a ratchet extension, twist the plug counterclockwise and remove. The oil will start draining. Once it has drained, replace the plug and tighten clockwise until snug. Replace with new lawnmower oil or SAE-30 weight oil.
Sae 30w
If its a push lawn mower. It has no oil release. The mower burns the oil. All you do is replace the oil when it gets low.
Any straight 30 weight should do.
SAE 10w30 full synthetic any major brand.