Thickest Oil?

What is the thickest Oil you can use in a 2001 Gsxr 600 "Safely". I know Harley guys like to run 20-50 but it says 10w40

I run 10 w50 repsol synthetic no problems

As you know, a multi viscosity oil starts out thin and thickens as it warms up. So, at start up you are getting good protection to the moving parts. Then, as it warms up, it gets heavier providing good protection under engine stress and heat.

I thought it gets thinner as it heats up

So whats a good oil to run then? My engine is more then broke in.. What would be a good thick weight to run?

Check the manual, typically 10W40 motorcycle specific oil. Some newer motors may ask for lighter oil as gaps between parts get tighter and tighter as they look for more power and come up with better techniques to build parts

Does anyone recommend a specific brand of oil or are they all good?

GETTING BACK TO THE OP's QUESTION The thickest oil you can run is that which is allowable by the viscosity chart in your bike's manual, for a given outside temperature range. But why would you want to run it? The best viscosity to use is the thinnest oil that is allowable by the viscosity chart in your bike's manual, for a given outside temperature range. That gives you the most power and efficiency, and you get NO MORE PROTECTION by using a thicker oil than what is specified. Generally, in most warmer areas of the U.S., 10W-40 is the best all around choice. If you live in an area where it never gets above 100 degrees, 10W-30 is an acceptable choice. Regarding recommendations for brands, etc.... there is a lot of good information in this section.

IMO 20-50 would work fine but in WI. I think 10-40 would be a better choice and maybe 10-30 for fall and winter.

have you ever hurd of anyone adding the lucas oil addative to it.... a friend putes in 3 qts of oil and 1 qt of lucas

I would never add lucas to a bike.