my oil sensor warning light comes on when i put key on (all lights on dash) without starting the engine but goes off when engines started, checked oil level and thats fine, the previous owner changed the sump..... any ideas?

also recently noticed it takes a few turns to turn over and when the engine is started it blows a cloud of blue smoke out the back..... once started its fine and if engines turned off it restarts fine with no smoke... any ideas

cars a 1.9 tdi 110bhp.

thanks