if it's oil in the exhaust, it will eventually burn off... but that smoke has a blue tinge to it.
if it's white and is more like steam (ie it dissapears quickly once out of the car) then it's coolant/water being burnt off.
I'd suggest by checking the coolant level, then going for a drive and keeping an eye on the temp.
say 40 minutes or so.
if it's left over oil in the exhaust, it will have burnt it off by then.
if it's coolant/water, it will keep doing it.
once it's cooled down, check the coolant level in the radiator again.
if it's less, then theres your problem.
***Opinion below***
Here's the thing right.
RB20 is not worth rebuilding.
therefore, it's not worth pulling the head off.
it's also no worth putting an RB20 back in if you are removing the engine.
so, here is what I did.
10L of distilled water.
1 bottle of Car-go Seal-up
Drop all the coolant in the radiator (instructions are available on this forum)
fill radiator with tap water - go for drive till operating temp
let it cool down
drop the water again
Fill with distilled water.
bleed system off (instructions on this forum)
top up as required.
Shake up bottle of seal-up
put it in the radiator.
cap on
drive for 40 minutes (personally I left it in there)
If it was steam out the exhaust (white) then it will dissapear within those 40 minutes.
when you have time, dump the distilled water and seal-up stuff
fill with nulon coolant and bleed as per normal.
problem fixed (probably temporarily)
In the end, you will eventually fork out for an RB25 or greater conversion.
It's been just over 700kms for me so far.
I'm about to dump the radiator stuff out of mine over this holiday period and put new coolant in.
I know it seems like a long process, but it's piss easy.
/myopinion