What is the difference between using 0w50 and 20w50 oil in a Mazda RX-7 FD?

I can explain the mechanics of oil viscosity ratings and how 20w50 is typically used in these engines based on the available information.

In a multi-weight oil rating, the numbers represent the oil's viscosity index, which is its resistance to flow. A lower first number means the oil has less resistance and pours faster ⁷. While 0w50 is not specifically covered, a 0W oil will theoretically flow much faster and with less resistance at colder temperatures compared to a 20W oil.

Here is how 20w50 and other common weights are applied to the FD RX-7:

• 20w50: This weight is highly recommended if you live in a hot climate, drive primarily during the hotter summer months, or are pushing higher power levels ². Because rotary engines and turbos create a high-heat environment, the thicker 50-weight oil provides robust protection at operating temperatures ⁷.
• 10w30 & 5w30: For cooler climates, winter driving, or near-stock power levels, the factory specification leans toward thinner oils like 10w30 ³. Mazda specifically recommends 5w30 for below-freezing temperatures ⁵.

Regardless of the specific weight you choose to run, the most critical factor for rotary engine health is changing your oil early and often. Fuel dilution is inevitable in the 13B, and as fuel mixes with the oil, it loses its lubricating effectiveness ². Keeping your change intervals short is more important than running a boutique oil grade.