Hi, Welcome to the Forum. 'too high a . .
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CliveAmen! Google is a source for a good feel of what is being written, but not necessarily how it should be written.
If I were you, I wouldn't be surprised at anything you find via Google. The internet is like the Wild West.
"Concentration" is a countable noun, whereas "viscosity" is uncountable. You can have a concentration, but you cannot have a viscosity. Hence, we usually say "too high a concentration", but it´s less common to say "too high a viscosity".