Bad valve stem seals or worn guides.
Blue smoke is OIL, Black is GAS, White is COOLANT.

Excess oil, not gas.

Probably valve guides, but could be rings. Often the engine is always
buring oil, you just don't see it when moving. Get soemone to drive it
while you follow in another car--you will probably see some smoke at
all times.

How many miles? How far between oil changes do you go?

Try adding some Lucas oil stabilizer. It may be a good short term
coverup (not fix...) but eventually the engine will need a rebuild.

Most often the cause is not changing oil when required.

I know people who do three year lease cars and don't ever, for the
three years, change the oil! Towards the end of teh three years they
will do an oil change so it looks good when they turn it in. If you
buy a vehicle that came off lease, you should ask for service
documentation to show all services were done.

With an 86 Nissan, no matter what, I suspect you have gotten your
money's worth. Try the Lucas stuff first! (I use Lucas oils in my H1
Hummer (diesel engine, diffs, hubs, etc. and I'm very happy with it.)