Much has been discussed on the topic of grease, oils and lubrication. There is no “best” choice here.
High temperature rails come pre-packed with heat-resistant “Krytox GPL 225 Grease” These greases are compatible with other PFPE/PTFE greases, but PFPE lubricants should not be mixed with other common types of lubricant. Note: The Krytox is expensive, it may be worth removing and storing as much of the pre-packaged grease as you can when you receive your high-temperature rails! Source
Standard Misumi rails come packed in “Kyodo Yushi Multemp PS NO.2” In the US it may be hard to source, but close alternative would be the AFB-LF from THK, any grade 2 lithium grease should be fine though. Source
Currently unknown which grease comes with LDO rails.
People are also reporting that you can use a light grease such as superlube 21030 or a oil, such as superlube 51010. Super Lube grease is noticeably quieter after switching from tri-flow PTFE oil to superlube grease. Superlube compatability sheet