When filling my car with oil I noticed a whiteish grungy deposit on the oil filler cap area. Anyone know why this is?

The gunk (iirc) is caused by condensation and is expected if you do a lot of short journeys. If you don't do short journeys it could be a sign of something more serious and is worth checking for oil in the coolant.

Chris

check and see if the coolant is oily or oily deposits can be felt around the top of the expansion/filler cap!

if you feel that it is greasy then the head gasket may have gone!