My understanding of the legal requirements when I was a lubricant field rep is that a manufacturer could not require only its brand for maintenance, but could write a requirements specification that others were free to access and attempt to meet. The OEM could also license approvals and advertise the competitor's product as licensed and vice versa. GM's "Dexos" Specification is a prime example. The OEM can require an oil vendor to pay for the license and prove their oil meets the spec. Some elect to do so, some do not (Amsoil is famous for not paying for licensing).
The user is obligated to show proof of change with correct oil, via a service garage or receipts form do-it-yourself. OEM's have deep pockets compared to their customers. Without proof of using an approved or meets-spec oil, you lose.