I switched over at about 80,000 miles with no problems. 88Cobra, if your losing the engine next year I'd throw in some stop leak and call it good (took a 4 cyld. Cherokee I had from a quart a day habit to nothing in 1,000 miles). Course, if your switching the engine why bother to use the synthetic?
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'95 Chrome Yellow GT ragtop, KN filter charger, Dynomax cat back, Accel plugs and wires, Chromed '95 Cobra R's (bought for $19.95, now 2 are warped!
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http://www.mustangworks.com/cgi-bin/moi-display.cgi?1883