I just changed my oil (to 15/50 from 10/30) and filter. I'm a bit over a 1/4 tank (last time it was full) and ready to add some denatured alcohol. How much should I add to approx. 5 gallons of premium?

Also is there any risk to the rubber seals with this trick? I will top off the tank right after the test.

Thanks in advance,
David

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K