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Hi Mary
If your engine hasn't been modified, you could use a mid-grade fuel (89 oct). Volkswagens reccomendation is to use at least 91 octane. I have a stock '66 with a 1600 single-port engine and have always used cheap gas in it. My girlfriend has a '71 1302 with an 1835 and two carbs. Her engine has much higher compression than mine, so she has to use 93 octane in hers to keep it from getting spark-knock.
What an octane rating tells you is how the gasoline will burn. The higher the octane rating the slower the burn. Low octane gas doesnt burn as much as it explodes. The explosion can be heard as spark knock. High compression engines use high octane gas because the extra compression makes the air-thus-fuel mixture much more volitile, and will ignite before the spark plug even fires.
As far as carbon goes, I think a nice, fast trip up the highway every week will keep your engine clean inside!
Hope this is some help.
yetibone
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