Blue smoke from your exhaust (tailpipe) is caused by engine oil finding its way into the combustion chamber where it is burnt alongside the fuel.
The 2 main causes are:
- Worn valve stem seals/worn valve guides
- Worn pistons/rings .. a worn engine.
And also a combination of both the above.
Valve Stem Seals/ Valve Guides
valve stem seals and guides Google Search
A valve stem seal is simply a rubberised seal that fits onto the valve stem. It prevents oil from the rocker box from trickling down the valve stem, and into the cylinder bore.
If the stem seals perish or split, oil can find its way down past the valve and into the combustion chamber. This is most noticeable at start-up when a cloud of blue smoke appears behind the car. The amount of oil lost is actually negligible. It only takes a couple of drops of oil to create that blue start-up cloud, which soon clears.
The valve guides are what the valves move up and down in. With age/wear (or infrequent oil changes..) the bore of the guide becomes slightly oval. This allows the valve to have side-to-side movement and oil can find its way into the combustion chamber.
There's different oil additives that you can try
valve seal additive Google Search
If an additive doesn't work, Cylinder head removal is required to replace valve stem seals/worn valve guides.
Worn engine ...
worn piston rings Google Search
Worn piston rings/worn bore allow engine oil up from the crankcase and into the combustion chamber, where it is burnt. There may be a constant faint stream of blue smoke as you drive, particularly noticeable when you accelerate. Your oil level will probably drop noticeably and there may be a 'burnt oil' smell when you get out of the car.
Fitting new piston rings can be a pointless exercise. Where the ring fits into the piston groove - that groove can become 'rounded' because of the flexing of the ring on the piston up and down movement. Fitting new rings onto worn pistons is pointless. You must replace the piston as well.
If the bore is worn - ovalled - because of the wear on the cylinder walls caused by the up and down piston movement which causes the wear, fitting new pistons is also pointless. The engine should be rebored. That entails new big end bearings and a cylinder head overhaul. On an older car it's often cheaper and easier to replace the engine with a second hand unit.