Go by exhaust gas temperature. You can pickup an EGT gauge and probe on ebay for around $50-$100. It won't be as accurate and fast as an O2 sensor but it will keep you from burning anything up.

Also, I'm pretty sure they don't sell leaded gas anymore so I'm assuming your adding a lead substitute. I think some of them are O2 sensor safe, I may be wrong though.

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320