Can I use WD40 to ease a sticking wooden sash window or will it make the wood swell? ?

No. WD-40 will make the wood swell up and the window will jam.

Rub the wood with a candle. Do not light the candle. Just rub the candle on the wood. The wax will make the window slide better.

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Yes I have done this. You'll need a stiff putty knife, no more than 1 1/2 inches wide. Put the corner of the knife where the sash meets the frame, you know, where the crack should be and drag the point along the crack chipping away the paint. This takes some muscle and some time. Have Fun

WD 40 is a water displacement solution, try the paraffin wax a light coat, or use a silicone spray it really helps. Outside of that it may be because of coats of paint you may want to remove the sash stops to plain the sash a slight amount use a coat of urethane on the exposed edges and reinstall.

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Paraffin (canning or candle wax), or Vaseline.

Soap will attract moisture and make the problem worse.

I've used paraffin wax on ours, careful though, it only takes a little! If you use too much the window won't stay up when you want it to!

Graphite is actually what you should use.

Second choice is some automotive grease.

Third and probably OK is WD40.

Your fine..........

honestly,auto grease??? try wax (rarrafin) or hand soap (bar)