Does sound like classic turbo oil seal failure.
Check if the exhaust is separated with an X or H pipe, sound like it is.
These early Garrets have poor rear oil seals and if the pressure inside the turbo core (crank case pressure) is higher than the pressure in the exhaust the fluid will likely leak. This is why it happens once warm and on idle, sometimes just the aircon being activated is enough to stop/stem it. There’s various reasons for this like poor compression, bad vacuum, exhaust design and even blockages in the turbo drains.
Check that the engine vacuum system is working effectively, not only is this functional PCVs but the restrictions in the cam cover tubes.
Mine did the same after decat, rebuilt the turbos and they still did it a bit. I then modified the vacuum so that there was more savaging and it’s been golden ever since.