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Hello I will help you with your question,
The factory fill is Castrol 363 which is a DOT 3 spec fluid with a lubricant added. The DOT 4 is the same fluid with a higher boiling point so they are compatible.
You should not use any product marked synthetic, they will not mix with the conventional fluids.
If this were my vehicle I would use the Castrol 363, if you wish you can install DOT 4.
Please let me know if you have further questions