I'm not familiar with the Giant seatpost clamp arrangement, but if it has a cylindrical shim between the seatpost and the seat tube, oil may be part of the solution.
I have 2 Dahons and a Tern that all have a shim. While it's true that oil between the shim and the seatpost will cause slippage, lubrication between the shim and the seat *tube* is helpful in developing enough compressive force to hold the seatpost in position. I use a very thin "smear" of wheelbearing grease on the outside of the shim. As the shim is inserted into the seat tube, some of this grease may ooze out; this must be wiped off so it doesn't get on the seatpost.
Another place where lubrication helps is on the cam of the seatpost clamp; this helps the clamp develop its maximum compression.
As noted, your weight is not an issue; I weigh 210 plus a heavy backpack, and have no slippage.