Whether it is mineral oil, synthetic oil, silicone oil or even fluorine oil, the use temperature should not exceed 300 C.
Recommend two practical methods for your project: 1, drying lubricating film. As you mentioned, the synthetic oil completely volatilizes around 250C, leaving a dry lubricating film, which provides lubrication up to 600C C. Among them, molybdenum disulfide and graphite are solid lubricants, and molybdenum disulfide is more effective in dry environment. 2. If it is OEM, the workpiece can be coated with high output and low load. The principle is that solid lubricant and organic resin are firmly adsorbed on the surface of the workpiece through special processes (such as Volkswagen engine piston coating).
The above two methods recommend products: Spanish Eagle brand BESLUX GRAFOL and American everlube functional coating.