There is no reason why an engine of this design needs to smoke, other than very very slightly.
The turbo's do NOT have seals, so will draw a certain amount in, but there are mods that can help here.
Yours is almost defo stem seals, OR shagged guides, much over 40,000 and they will be shagged, they were chronically bad, but were shit material to start with from cosworth.
I use COLSHIBO guides, each one hand machined to size but they will never need doing again.
There will be pressure present in the form of crankcase pressure, the oe breather was emissions friendly unfortunately not power friendly.
Basically on a turbo car you can't have enough breathing, any pressure that builds will allow/help oil get past the rings.
A 10W60 would be my advice to use, the other problem with these cars is the stupidly tiny sump capacity, they really should have had a air to oil cooler as std(a stat'd one), but modine was cheaper!!, the oil simply gets too thin when ragging it.
All you can do is make sure there is a decent breather and it is clear, there is a different breather set up available called evac sump, where it uses the exhaust system to suck the pressure out, been used for years not used here, but works perfect.
Now before anyone says the exhaust is under constant pressure, it isn't, between each power stroke going down the exhaust a vacuum, or rather low pressure area forms behind it, the yanks use these with anti backfire valves, so in the event of a backfire it doesn't blow the sump off!!
tabetha