My son has an S-10 with an Isuzu manual 4-speed. We replaced the Dextron II ATF with a synthetic. The transmission used to grind the synchro's every once in while with the Dextron II but now with the synthetic is grinding when you downshift from fourth to third. Could I go to a heavier gear lube in this transmission to reduce the grinding until we can replace it or rebuild the synchro's?? The manual calls for ATF.