GMC 1995 Suburban Owner's Manual page 296

Before closing the hood, be sure all filler caps are on properly. Pull down
the hood and close it firmly.
Cleaning Your Diesel Engine
I
NOTICE:
If you spray or pour water or any other liquid on your engine
when it is warm or hot, or when it is running, you could cause
serious damage to it. If you ever clean the engine, clean it only
when it is cold.
Engine Oil (Gasoline Engines)
It's a good idea to check your engine oil every time you get fuel. In order to
get an accurate reading, the oil must be warm and the vehicle must be on
level ground.