GM designed the LS1 around Synth oil. due to heat copmpression and the block and heads. Synth will leave residue and deposits in other type of engines. Synth is a way better oil its viscosity can be "tuned" persay for the engine chenically. AS stated above Synth under high heat conditions ir turbo will leave residue and clog the turbo, as i stated they make special turbo oil. Weight really is just for weather conditions you havnt tapped the keg with just 5w30-10w50 there are so many types of oils you couldnt list them all. These are just the most common weights. The honda hybrid uses a 0 weight oil. it looks like brown water. Like i said before it depends on your engine, engine material, performance peak of yor engine, and the amoung of heat the engie sees. Synth can be good and bad dependiung on your application. If you start with Synth stay with it and vis versa.