Assuming this is for 2-3 seconds on cold starts after the car has sat,
overnight...probably minor valve guide seal leakage.  Typical as L98s
age.  (Some new L98s did the same.)  When my L98 sat for over a week it
always threw white smoke.  Now that it's being driven 3-4 times a week
there's less white smoke.) (58K miles on the engine since new.)

Don't know if the '90 used a PCV filter (the '89 did not.)  If so,
replace it along with the PCV valve.

Otherwise:  Rule out gasket -- Any oil in the coolant?  Any coolant
consumption?

After that, pull the plugs (number which cylinder each came from) and
visually check.  If you don't know what you are seeing, have a trusted
mechanic look at them.

Take a critical look at oil consumption.  If it's over a quart every 2K,
consider the type of driving and maybe see a mechanic.