An oring with a thicker cross section might do the trick.
In an ideal world you shouldn't need or use sealant with orings. Problem is I've never been to this ideal world. A little judicious use of silicone won't hurt anything, and might solve the problem.
is notorious (in my experience) for using orings of insufficiently thick cross sections. When I rebuilt my TAMD41, I used OEM Volvo seals
and my tappet covers, torqued to Volvo spec, leaked badly. Copper siliconed them and tightened to my intuitioned torque (tight enough) and they didn't leak anymore. The w/pump oring leaked too, I added a homemade gasket (and left the oring in place as well) and no sealer and stopped that too.
I assume that it is oil
that is leaking?
You might check the crankcase pressure also. If it's too high it can cause oil