Blue smoke from exhaust - but only after oil change?

[Goochie]

We have a 1997 1.4 16V Corsa that we use for taking the dog to the vet and taking stuff to the dump etc. We've had it for 4 years and in that time it's done just 6k miles and I changed the oil and filters etc. along with the cambelt when I bought it.

Last week I was bored so thought I'd change the oil and filters - It has done 106k

Used Halfords own brand part synthetic oil along with their own filters. Checked all other fluid levels and they were all fine. Nothing concerning under the filler cap or on the dip-stick either.

However, I have noticed that upon starting the engine from cold, there is now a noticable puff of blue smoke (large enough to see it in the mirrors, let alone from outside the car) - I'm sure it diddnt do that before?

Any ideas?

[Goochie]

It was 10W/40 that I used. Not noticed anything whilst driving, certainally nothing once it's been for a drive and then left to idle.

Its not over-filled either.