I'm with Ken on both of these:
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Best oil Additive--Schaeffer's #132 E.P. Oil Treatment
Best Fuel Injector Additive--Schaeffer's #131 Neutra, and it's less than $4 a bottle.
**It's also less than $3.00
per bottle if you buy it in bulk (in 1-gallon containers) and then roll your own 12-oz bottles.
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I've been running Neutra 131 in the gas for a while now, but I just added about 5 ozs of 132 to the oil approximately a week ago. That's in a new 2003 Toyota ECHO running with 5-W30 Mobil-1 SS oil and a 3-oz maintenance dose of Auto-Rx. The ECHO has a small 4-cylinder engine, so jitter is apparent, and is passed through to the steering wheel.
The combined effect with the Neutra 131 and the 132 Oil Treatment is a smoother-running engine (better than new). It's mostly noticed when the car is in drive, stopped, with the engine idling. The 131 reduced this jitter by about half and the 132 seems to have nearly removed the rest of it; for much of the last week the steering wheel has been almost completely quiet during in-gear idle.
I was a bit surprised that the 132 made as much difference as it did for a couple of reasons:
1. I figured that the Mobil-1 SS, being a top-rated synthetic, would already have kept friction from the oil down to a minimum, negating the need for the 132.
2. The Neutra 131 gas additive had previously made quite a difference, so I didn't really think there was much room for further improvement.