GS engines run at such high oil pressure. I measure 120 PSI on a cold engine at 2000RPM & oil pressure @ 70 deg C is still 80 - 90 PSI at 60Km (about 2500 RPM).

There are also so many places for oil to leak from.
I have yet to see a GS that doesn't at least leave a few spots.

Some of the main problem areas I found have been due to a different technique of diagnosis. Oil leaks really annoy me so I tried this:

Simply remove the engine & sit it on some stable metal racking supported on say, some bricks.
Take off as much of the tinware as possible inc. the fan cowling but refit the fan & front nut. I noticed that some engines don't have a belt stay guide on the crankshaft so this may be important!

Use a container of fuel with a fuel hose (get it as far away as possible) & connect to the fuel pump.
Remove the battery & coil from the car & connect so as to activate coil/ dizzy.

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If I've remembered everything, you should be able to crank start the engine now.

I forgot to tell you how noisey this is. I first did this in a Noosa residential area after being failed for rego. So be sure to do it at 7am on a Sunday morning.

If it's misfiring you may find it dancing around a bit so stay with it. After 10 minutes it should be getting pretty warm (no cooling) so start to look for leaks.

Look at the front & rear crank seals. The newer seal types (with spring) can inadvertantly be back to front, ie front (clockwise) in the rear & will tend to throw the oil out a bit. If in doubt just replace, especially the bell housing end one (hard to get at in the car!).

Pay attention to the oil return pipes under each head, Although there's no pressure here, the buggers seem to leak if those "O" rings aren't an "O"!

In my first case I found the dipstick to be causing most of my grief. The nut holding the whole sharbang in was loose. I knew it was loose when it was in the car but didn't think it would be a major reason for failing rego.

The cam shaft seals always (even brand new ones) leak a drip when the engine has stopped. Just have to live with that one. I've never had a leaking oil cooler or the seals associated.

I always use that aviation black gasket goo (with the little paintbrush in the lid) on both sides of all the tappet cover gaskets & have never had trouble with them.
Can't say I've had any leaks from the rocker shafts (can't remember what you call them, the shafts that the tappets ride on) seals (the alum ones).

This is probably a bit dangerous but it worked for me & I had about 6 months of minimal oil leaks before it went back to it's old ways. Good thing about QLD is you only need to prove roadworthiness for 30 minutes & then she's right for life if it doesn't change hands