I’d like to get into a routine of doing a drain and fill every 15k miles on our new Mini (yields about 3 quarts). Fluid spec is JWS 3309 for the Aisin transmission. JWS 3309 is nothing fancy (mineral). There are many synthetics that are “recommended” for the JWS 3309 spec. Aisin or Toyota branded JWS 3309 fluid is about $6 a quart. What would you use? I’ve Castrol come up a couple times on various forums.