As I remember an m5 tapp (5mm) is what you need (the hole should be 4.2mm ish to tapp out to m5) ,I think mine was already threaded but its long time ...

Just send the cash to the usual place

The silicone oil must aid the air seal but not be so viscous that it adds friction ,if your talking about rc shock oil then you should be fine but if ...

I tend to use black moyibdomen grease on the gear teeth and regular mineral grease on the axles ,while there I'd lube the piston with silicone oil not...

Yup its fine .