This has been brought up a couple of times and a search of the forum might bring some good information. Hoppes works really well, try not to get any on the grips, as any oil is not good for them.
I like Breakfree for lubrication and oiling, although I also use Hoppes to clean.
On your left at
www.lugerforum.com is this link:
http://www.lugerforum.com/geninfo.html and there are two articles that talk about oil and rust prevention, along with other great info!