MBTE is being phased out nationwide. Everyone knew the stuff was water soluble when it was implemented, but the idiots went ahead with it anyways. Clean air at any cost, right?

Here's the deal:
Oxygenated fuels contain less energy.

Cars with modern sophisticated feedback induction systems simply inject more fuel to compensate for the air/fuel ratio change.
Cars with primitive carbs may pollute somewhat less due to a leaner mixture. The cooler winter air is more dense, so the mixture is getting 'optimized' with oxygenated fuels (in a crude way).

Since 'dumb' carbs are becoming rare, the whole matter is rather silly, and very costly to implement in the supply chain.

Now the mandated additive is polluting our drinking water. What will that cost? Regulatory meddling at its finest...

Anyways, there should be near zero difference in your emissions test. Just make sure the motor is fully warmed up so that the cat is hot, which is a seasonal factor to some extent.

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1986 300E 5-Speed 240k mi.