you need a 'polyurea grease' unfortunately seems very expensive
eg CASTROL FIRETEMP XT2
Castrol - Optimol Firetemp XT2 - High Temperature Grease - Geo Broughton
quote for future ref when the link is dead:
info:Castrol Firetemp XT2 is a high temperature grease for rolling and sliding bearings at normal to medium peripheral speeds and high operating temperatures. The fully synthetic, thermally stable structure guarantees long relubrication intervals at maximum operational reliability.
Castrol Industrial - High-Temperature]
found this which is affordable £5.24 so I would probably use it:
http://www.draketooling.co.uk/Produc...eases/MOL20005Molyslip EHT: Extra high temperature grease.
Bentone grease with MoS2 and graphite. Formulated to ensure that even if carbonisation occurs the residue will remain friable, with the MoS2 and graphite present to maintain lubrication. Temperature range up to 250°C.