First, IMO oxycodone isnt even a high. You just feel euphoric but it doesnt really alter your conciesness. Many other people like it though, so Im not gonna tell you not to do it.

Take about 20 mg to start off and wait an hour. See how you feel, and if you want more, take more. Im not sure how much it would take to overdose but I have taken 40 mg without bad effects so I wouldnt go above that. You most likely will be fine and dandy with a low dose of 20 mg.

As for guaifenesin, there are no known side effects. Its mostly safe as long as you arent downing 1000's of milligrams. Youll be fine with taking only 400-800 mgs which would be what you are taking with 20-40mg oxy.

Have fun. Make sure you measure it out right and stay below 40 mg just iin case