Sorry, I meant blow into the oil separator, not the oil filler. You need to open the filler cap to let air escape. That's to check if it's totally plugged. If the oil separator is marginally plugged and you're suspicious then you may want to remove it for closer inspection and attempt a cleaning.

The suck down test at the oil filler doesn't seem to work as well with the newer engines for some reason. You used to just loosen the cap to see if it rattled. Now you need to cover the opening with something like wax paper or plastic cling wrap to notice any suction. I no longer bother, but I do check my flame trap regularly. The flame trap is easier to see and get at with the TB removed, also of course accessing the oil separator box. If you've got an EGR valve mounted in front of the box and blocking access then getting that out if the way can be a real pain and if you're not careful may split the tube routed around the back of the engine, a part that is reportedly unavailable. I remove the TB anyway for a thorough cleaning and inspection at every oil change anyway so it's easy to reach underneath the manifold to restrain or reseat the flame trap hose on the oil separator. If you do it regularly the flame trap and PCV hoses don't get welded in place. The small plastic nipple on the flame trap housing gets brittle and is easily broken if you're not extra careful..

For seal conditioners in the oil, I wouldn't use them on newer gaskets as tge softened seal wears faster, but I'm willing to change the front seals more often if it extends the life of a worn rear main seal. As for repositions the rear seal on a less worn section of the crankshaft, if you can set it slightly deeper rather than shallower. There should be a lip at the back to prevent pushing it through. As for brand of seal conditioner, I forget which one I've used, perhaps Lucas, but any reputable brand should do.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now