Bear bubble balancer ??

Does anyone know what weight/type oil and how much goes in the top of a Bear bubble balancer? I thought there would be a level line but it doesn't have one. Thanks.

 

ATF. If it runs on the floor you put too much in.

 

As I remember, it was gear oil used in a bubble balancer.

 

Thanks for both answers. I doubt it really matters, but wanted other ideas.