I did a bit of searching on LR and Cobra replica forums last night. The general concensus seems to be don't mix the two, and do a thorough flush if changing over. Halfords apparently sell a ready diluted OAT which will allegedly mix with glycol based coolant, but I wouldn't risk that. OAT is red/orange, so it's easy to identify. It is also good for five years, rather than the two specified for glycol coolant. Having said that I don't know anybody who routinely changes antifreeze at two years, any more than changing brake fluid regularly!!
Airlocks? Plenty about this on the Cobra forum, but my MGBV8 didn't suffer with airlocks, even when filled from dry. The only thing I had to do was bleed the top heater hose which was the highest point on the system.
