I used Castrol GTX 10w40 winter, 20W50 summer in an old KZ650. Original clutch still working fine with 160,000 plus miles. However even though oil was changed religeously since new every 2000 miles, by 160,000 it was full of sludge. I'm not talking a thin layer, I'm talking I went to drain oil that last time and 8 oz of 90wt sludge oozed out, the rest of the 3.5qt was permenantly caked in the engine with the previous. Castrol does not like a worn engine. Maybe no oil would, but this was an exceptionally bad example.