I think you are missing a few things here IMO. First what is an "older model"? Second the change interval is not the only consideration depending on how old the car is and the type of motor. What oil type are you talking about? Why is the 3K oil change figure appropriate after you define what an "older car" is?
If you have an OCI and feel it reliable and it is made for use with your oil (conventional or synthetic for example) fine. Also what filter are you using. If you don't have an OCI then there is usually more than one recommended change interval depending on driving environment which all may change depending on the oils and filters you use compared to what the factory is using to obtain those figures. For the comment about "most newer models use 5-30" have you seen what most of the European car viscosity requirements are? Finally viscosity can change if there are environmental variables such as extreme heat and cold.
Those are some thoughts that come to mind for me anyway.
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