Update...

Kent at ************** says to use Permaflex Aircraft Sealant on paper and cork. That's what I am doing.

I know not to use gasket sealants on head gaskets but what about other areas of the engine? I am installing my rebuilt turbo this weekend and have several seals that need to go in:

Oil Line Seal
Oil Line Return Seal

Everyone seems to have their opinion on this, I'd rather hear from someone that's been doing this for a while. Do I need to use sealant after prepping both surfaces and installing a brand new gasket?

In my work, I decide by what kind of sealing is needed by what kind of surfaces and gaskets are involved and the condition of the sealing surfaces.
It's a case by case basis with me.
It also depends on if I'm trying to seal (hold back or keep out) Coolant, Oil, Fuel, Air, etc.
Or a wet stud going into one of these fluids ?
Has / is this mating or joining surface corroded or had corrosion ?

Sealants can be Aviation seal, Gasket cinch, Right stuff, RTV silicone, etc.

Are you referring to an "O-ring" seal ?
Or a "shaft seal" ?

And the list can go on and on.

Post some pictures next time of the exact application and installation.

I was a diesel mechanic for 18 years. I went to trade school for that around 1975. If there was certification back then I did not know about it and my Instructors never mentioned it.

This is a quite old thread so we likely cannot ask him what was meant by Oil Line Seal and Oil Line Return Seal.
I looked up the vehicle- 79 300D, 83 300D and 85 300D. The US 83 and 85 would have a turbo.

Anyway at the top of the Turbo is an oil inlet and it uses a paper gasket and there is a 2mm hole on the turbo. What is important here is you don't want to use any gasket sealant that is going to mush out and cover or restrict the hole when tighten the parts together. Not a good place to use silicone sealant due to how thick it is. The mentioned Gasket Cinch/Gasga Cinch would be good for that because it is a thin rubber cement that tends to soak into the paper of paper gaskets. If you carefully use a thin coat of it is not going to squish out into the hole.

The other end of the Turbo Oil Supply line has a banjo bolt and 2 crush washers that don't need any sort of sealant on them.

The Turbocharger Drain (oil return line seal) also has a paper gasket at the top and while the hole is larger like 1/2 inch you would be better using a thin gasket sealer like Gasga Cinch.

Also on the Drain tube is in 2 sections. The upper portion has the mentioned paper gasket but the lower separate section has an O-ring on each end that needs to be lubed

Where the Turbo Drain Tube goes into the Block there is a rubber Grommet that is supposed to seal all by itself if you do that lube it inside and out to help with installation.

Being a pessimist I lubed the inside of the Grommet but put a light coat of silicone sealant around the outside part that goes into the Block above the Oil pan. But that was a personal choice.

In the attached picture is the turbo drain paper gasket, O-rings and rubber grommet. The oil inlet gasket is similar to the turbo drain paper gasket but smaller.

In the next pic of a diagram the green arrow points to the Oil Inlet Gasket. Sorry I don't have a better picture to post.

(The turbo drain grommet area is a common place for a leak on old Mercedes.)

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84 300D, 82 Volvo 244Gl Diesel