You can't reset it without the right hardware/software. The ECU stores it's fault codes more or less permanently, even disconnecting the battery doesn't help.
It's unlikely to be the oil. If you are in the UK there are a number of independant Land Rover specialists who will take a look at it/reset it for a minimal cost. I use Maddison 4x4 and can recommend them if you're within reasonable travelling distance.
If you're outside the UK, the same applies but you might have to look a bit harder !
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