I tried placing a block of wood and tapping it with a hammer but no movement at all.
Could I add some oil through the hole for the stem bolt.
Try using a good wack rather than a tap. Use your common sense though, you don't want to break anything.
Yes, dripping some oil inside the stem from the bolt hole may help loosen the expander/wedge. If you have some KROIL OIL that would be the best thing but any kind of penetrating oil will help and let it sit overnight.