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Which type of engine oil should you use and what viscosity? Freshly built engines should get a conventional, non-detergent, heavy-duty SAE 30 weight oil for proper break-in. Once the engine has passed 500-1,000 miles, you can run Mobil 1 10W30 or 10W40 synthetic along with a ZDDP additive. If synthetic is beyond your budget, choose Castrol, Valvoline, or Mobil 10W30 or 10W40. High-mileage engines that have had a diet of conventional oil need to stay with conventional oil, either 10W40 or 20W50.