meet specification is only one part

keeping that specification until next service is the real story

no la, oil specification only based on new oil not used oil

cheap synthetics are cheap for a reason , one of which is, they take a lower viscosity base oil like 20 or 30 and build it up with viscosity modifiers or improvers to make it meet the wanted viscosity

but there modifiers get broken down by shearing forces and the longer it's used the thinner the oil gets , the Golden rule always applies, you get what you pay for

just check and see, the certifications don't guarantee the entire service life of oil, it only says yes at this point of testing the oil meets the required specification

practical wise , also doesn't make sense to test used oil, no one uses second hand oils and since it being changed anyways why test anyways.

check all those extended service life oils are all self claimed, no one will certify it officially