Oil Pastels and Solvent for Landscapes

Trees on a 3" x 5" picture are difficult for me when I use the Sennelier oil pastels.  So I used solvent to make the oil pastel more liquid and, with a fine brush, drew the small branches of the trees.  I used the same method to draw in the grasses and reeds.  I had a small paper on which I smeared some oil pastel and then used a brush laden with solvent to liquify the oil pastel and draw the branches and grasses.