I never buy oils from a dealer but I do buy OEM filters mostly because I am too lazy to try and cross reference them AND they are a relative long term item so I don't feel the pain very often.
I don't buy dealer oil, because lets face it none of the manufacturers (tractors, cars, motorcycles etc) make their own oil and only spec that the oil meet certain API classification which you can get from anywhere. I do stick with popular brand names. I like the Shell Rotella T6 oil for all my engines because I can use it in any engine I have whether it is diesel or gasoline, so I don't have to stock a bunch of different oils.