Clear["Global`*"]
f[x_] = Log[10, x + a/x - 2]
FunctionDomain[{f[x], a > 0}, x] // Reduce[#, a] &
USE
In[22]:= Clear["Global`*"]
f[x_] = Log[10, x + a/x - 2]
FunctionDomain[{f[x], a > 0}, x] // Reduce[#, a] &
Out[23]= Log[-2 + a/x + x]/Log[10]
Out[24]= (0 < x <= 2 && a > 2 x - x^2) || (x > 2 && a > 0)
The answer above is incorrect
the correct answer is as follows:
(0 < a <= 1 && (0 < x < 1 - Sqrt[1 - a] || x > 1 + Sqrt[1 - a])) || (a > 1 && x > 0)