if the oil is crap then there's that risk
but if the oil is good and have a high HTHS (high temperature/high shear) rating, then it is NOT a problem
crap oil shears out of grade to a lower viscosity, and will also break down at higher temperatures... so in otherwords, a low HTHS rating
shearing happens due to mechanical stresses to the oil, such as with flat tappet valvetrain or tight bearing tolerances
modern engines run hotter than ever, and ACEA A3/B4 requirements as well as ILSAC GF5 requirements have strict requirements for oils suitable for these engines
used oil analyses by many people have shown that after 12000km, a good quality but affordable (ie: not boutique brands such as Motul or Red Line for example) still have a high viscosity and high base number as well as very low wear metal count... this is indicative that the oil has life left in it.... most analyses of 16000km drains have shown that the oil still have a safe margin left (good till 20000km), even for those run in extreme climates... but for the sake of peace of mind instead of trying to stretch things too much, 16000km is a good safe number for quality oil
