I know this is an old school trick, but I recently found out that a lot of younger DIY mechanics don't know about this one. So I figure - what's the harm in re-stating the "obvious"?
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Changing the gear and differential oil on most cars is, more than anything else, a waiting game. Filling a gearbox usually involves spending 10-20 minutes watching high-viscosity oil slowly snake its way down a long piece of plastic tubing into a fill hole buried in the depths of your car's undercarriage. It's a mind-numbingly dull exercise that makes you wish you were watching paint dry.
Fortunately, there is a way to make the process go a lot faster. Take your bottles of fresh gear oil and soak them in a bucket of hot water for a few minutes before you start your oil change. The oil will flow much more quickly and you will have your gearbox filled with fresh oil in a fraction of the time that it would normally take.
This trick is also useful for those of us who use high-viscosity oils to protect our highly modified engines. Your Shell Rotella 15W-40 diesel oil will flow like a 5w30 synthetic after a minute or two in a hot water bath. No more stopping and waiting for the funnel to empty.
And speaking of paint drying, you can also use this trick to pre-warm spray cans before painting. You will get fewer splotches and blots when you spray, and you won't need to shake the can for as long to get nice even coats.
Just remember that hot water means hot water from the tap. Don't boil your spray cans on your stove, unless you want to end up with a massive explosion followed by a huge multi-colored mess to clean up.
