Over-tightening of bolts is a common problem on all things mechanical. I'd recommend a ~$20 clicker Torque Wrench (ft-lb and inch-lb. are available; ft-lb is more commonly used, but having both means you're covered) from
Harbor Freight. They claim to be within 4% of stated torque, which is so much closer than guessing...
The oil drain plug is one of the more common things to damage by over-tightening (usually requires a new oil pan and bolt...). A Torque Wrench will allow you to get it tight, without damage. Same applies for all other bolts on the bike. The FSM has most torque values listed, so you will know they're tight and right. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII