can this level withstand 300 degree temp., I lay ashalt and need to check the slope ?

I took the heatgun to mine to answer this. The wood part browned at around 340. The plastic end caps started to bubble (very small bubbles) between 275 & 295. definitely softened and started to deform at 320. So, will it take 300 degrees? I'd say, likely yes if it's not going to be left on the surface for long, more than say a minute or two. The plastic end caps will be the first part affected by the heat. The same kind of plastic appears to be used around the vial windows. The vials are held in a clear plastic insert of somekind and appear to be locked into place or held in by the brown plastic insert (same kind as used on the end caps). The assembly may be glued in, as I didn't try prying the brown plastic around the vial up. If it is glued in, the brown plastic is just a trim ring. If it were to lay on it's side for the same length of time, you'd probably experience a little plastic deforming in that area (and maybe the vial window itself). If the brown plastic did melt out and the vial assembly still intact...one could always apply a little high temp RTV in place of it. Standing upright like you'd normally use it...you'd likely have no problem. Maybe give it a minute or two to cool down between placement on the hot surface. Hope this helps with your answer or decision.