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After the break in, why not change to synthetic after a few thousand miles?
You need to run conventional for the first 10-12k miles. That's about how long it takes for all of the engine wear surfaces to completely wear into eachother. Synthetic oil is such a good lubricant that it prevents the wear surfaces (especially the cam and piston rings)from breaking in, so using it before this period of time is not recommended.
Us guys out here in the southwest need the 20w50 oil. Never seen 20w50 look like tar though in the last 30 years, maybe you were looking at 85-95 for a rear end!
RE: Zinc in oil
