Mixing them will reduce the lubrication properties that you expect from synthetic oil’s.
It can also be dangerous if the chemicals in the two oils mix or break down and thus turn it to sludge.
Normally a small amount is fine in an emergency but the general rule is DO NOT MIX THEM !!
Going back to original point of the post - I’m with Mike Lane - it sounds like piston slap - the engine will also probably sounds really rattly when cold and then shut up at about the same time the smoking stops
As long as you drive at low revs whilst the engine gets warm - i.e. less than 4000 until you get to 70 degrees c, then there should be no shortening of the life of the engine. If you thrash it cold with piston slap, then you will get through liners and piston rings like they are going out of fashion.
If you want to talk it through, always worth getting hold of Dave Andrews and having a natter
I SUPPOSE I WILL HAVE TO BANG A LOAD OF MILEAGE ONTO THE CAR AS IT HAS TAKEN ME 5 MONTHS TO DO 1200 MILES .
THE CAR IS USING 1 LITRE OF OIL PER 600 MILES
My last car had a bit of a water and oil issue…I ended up running the car through europe for a week on Evian and Shell Helix Ultra. Expensive week! My Elise smoked a bit when new. Then that died down after a while. Was never a problem again after 1000 miles or so.