The best replacement for thinner is reducer.

The best alternatives to thinner are rated as following:
★★★★★  reducer
★★★★★  mineral spirits
★★★★  turpentine
★★★★  acetone
★★★  paint thinner
★★★  more thin
★★  white spirit
★★  thicker
★★  thiner
★★  lacquer thinner
★  thicker oil
★  retarder
★  anticoagulant
★  mineral turpentine
★  paint remover
  solvent
  thicker head gaskets better
  varsol
  paint stripper
  denatured alcohol
  terpentine

Your alternative for thinner is not listed? Suggest!
Speaking about thinner, one probably shouldn't forget about mineral spirits.

Other search terms related to thinner and their ratings are:
★★★★★ mineral spirits
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Google Trends

Scale is based on the average worldwide traffic of thinner in all years. Click on graph for more.
thinner
reducer
mineral spirits