Try this, drive it till hot, park on a level spot, shut it down. Come back in 15 mins, start it up, check it. It will still be hot and you'll be able to get an accurate reading since the fill tube will be drained by then. Been doing it this way on my 87, works great.

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Brian
87 300Dturbo 180K #14 head still running R-12 SOLD 12/2017
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10 Sprinter 3500 chassis with a Class A Winnebago on it. 56K