We have a 1988 Mazda RX7 with an odometer showing 81k+ miles. We don't know whether it rolled over, but it has at least this many miles. Based on the condition of the car, I'm inclined to think it has 81k miles and not 181k miles. We bought the car from a dealership that had no service records. A rotary mechanic inspected it before purchase and said it was in great mechanical shape, and we drove it from FL to NC, and aside from a new battery, it was flawless.

We put in Idemitsu 20W-50 Racing Rotary Engine oil. Then we learned that some people at least, say not to use synthetic oil. At 80k miles and 35 years, the engine has been broken in. We can use conventional oil on the next change, but is the synthetic so bad that we should change it before driving it again? We put on about 30 miles on the new synthetic.

As an aside, at $100 for 5 quarts, I don't mind using conventional.

Thanks, Woodsman

  • Welcome to Motor Vehicle Maintenance & Repair! May 8, 2023 at 9:57
  • How many digits are there on the odometer? Only 5?
    – JimmyJames
    May 8, 2023 at 15:55

1 Answer 1


For someone to state "don't use synthetic" leads me to believe they have an old school mentality. There is absolutely nothing wrong with synthetic oil (aside from the expense) and may actually provide your rotary with some added protection. Using it will cause your engine no damage. As long as the oil is of the recommended viscosity for the car, don't worry about it.

As I stated, about the only downside for using synthetic is the cost. Considering rotary engines use oil by design, you have to figure you're going to be using more than the 5 quarts between oil changes. It may be more economical to use regular dyno oil which is of the correct viscosity. On the other hand, if your pocketbook doesn't flinch at this, my suggestion is to continue using the synthetic, as it will provide better overall protection for the engine.

  • An engine doesn't "know" if it's using synthetic or conventional oils. The compounds are the same. The difference is that synthetic oils have fewer "impurities" which are hydrocarbon compounds that don't perform as well as the desired oil molecules. Synthetics, being built up from simpler molecules, don't have these and are therefore "purer" in that they contain almost nothing but the desired oil molecules and additives. So they tend to last longer.
    – jwh20
    May 8, 2023 at 15:55
  • You are right the engine doesn't know the difference in the long run. The difference between dyno and synthetic oil was described to me as the molecules being more consistent in synthetic, whereas with dyno, you kind of get what you get. May 8, 2023 at 16:22

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