Do a search for diff oils and you'll find loads of info. Basically fill to at least 80%. (Personally I fill to 100% and have never had a problem but some people will advise against this). Shock and diff oil is the same stuff but diff oil in general is a higher viscosity. You can also mix oils. I,E 50mls of 200wt and 50mls of 400wt will make 100mls of 300wt.