I have 44,000 miles on my car and I put the 0w30 german castrol in last oil change. Question I'm asking is how long can I keep putting this weight in my car?

 

I'd say you can keep putting that oil in your car for as long as you like...

Now, if you're talking about frequency of oil changes, I'd probably use 6k miles/6 month intervals for changing oil and filter... and change at whichever comes first.

Any good synthetic like German Castrol, Amsoil, Redline, Royal Purple, Mobil 1, etc. should be able to make that change interval just fine. Just use a good oil filter at the same time (K&N, Amsoil, Wix, Napa Gold, etc) and you're good to go.

 

The only way to know for sure is to get an oil analysis done.

http://www.blackstone-labs.com/