Like others say, your OCI is too long. The varnish is indicative of this - you're getting oxidation of the base oil, additive(s), or both. In my experience, Rotella's are set up for diesel OCI's. I think they're up to 15,000 miles, but they have 12 gallon sumps. Your Tacoma V6 is what - 1.5 gallons? Rotella is heavy with DDI for soot suspension as well, that you don't need for your small gasoline engine. If you're running a mixed fleet, sure, but then just tone down the OCI. Way down. I'm using PUP in my gasoline engines. Haven't opened one up yet.