Hi there
when the smoke is bad + engine cold pull plugs out .one broken ring will show up as residue if its as bad as you say.One bad sealing guide will do the same,but on removing head to fit rings its near impossable to strip without noticing what ports are throwing out oil.Bad sealing rings should stick out a mile with head off .
I have almost paid for rings from an engine reconditioning co who reckoned
chrome rings in a chrome bore was fine .I refused to pay for them
Something like wrong parts,wrong set ring gap,faulty head ie guides exess play through machining or wear is always poss.I have had a head leak oil extremely bad into bore cos the head wasnt skimmed right.Fine grooves just does not seal-worse with a metal gasket.
If the garage does not find fault then try locate which cyl is good [no oil] +which is bad?+post back <_<