但是合成机油在老款车中是用不了的，不是机油不好，而是发动机吃不消，洋葱也说，合成机油在进入发动机以后会膨胀，一旦用了就不能换回去。就好像人生病了，本来要对症下药，结果您一下就给病人吃的是大补的，不仅没有益处，反而病人体制接受不了了。
这已经是21世纪了，30年前可能这样，但是在今天，几乎没有神马车不能用合成机油。“合成机油在进入发动机以后会膨胀，一旦用了就不能换回去”这纯属胡说八道。机油就是机油，你随时可以换回来。
https://www.theglobeandmail.com/glo...to-synthetic-oil-a-good-idea/article10582654/
It's a bad idea to flush an engine just because you're switching oils – synthetic and conventional can mix just fine, says a BCIT expert.
"A flush here is not required in either vehicle," says Eugene Eng, professor in motive power at BCIT in Burnaby. "The two oils are miscible, meaning they can mix together in any ratio."
Engine flushes aren't really for flushing oil. They act like drain cleaners, washing carbon deposits out of your engine. In an
engine flush, the technician removes a bit of the oil, adds the flush additive and runs the engine. The chemical loosens all the, er, crap. It gets washed into the engine's oil and caught in the oil filter. Then the oil is changed normally.
"It was once commonplace to add automatic transmission fluid to oil and run it before oil changes, as the detergent in the fluid would serve the same function," Eng said. "It's absolutely not a recommended procedure, but the idea was been around for decades."
Synthetic and conventional can mix, so you don't need to get all of the previous oil out.
