The following may help if your Mitsubishi's 4cyl has HLA's / hydraulic lifters.
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Would the noise be better described as lifter ticking? Does your engine have HLA's vs a solid tappet. Mitsu's with HLA (esp early 90's) are somewhat notorious for making some lifter noise at startup before they are fully pumped up. The older lifters have an extremely small orifice within the lifter body that tends to get clogged with dirt and deposits, which means that air gets trapped inside.
I might suggest that you try Auto-RX or another cleaning products of your choice. FWIW, I have a brand new '02 Mitsubishi with a 4cyl (4G94), and it also ticks away for about 20 to 30 seconds before quieting down.
From information recv'd on another forum I belong to, the Mitsu's HLAs seem to be overly sensitive to oil that is thin or dirty. The 40W may not have helped the problem if the lifter(s) are already dirty or clogged. A relatively thick oil when warm may have a problem flowing through the partially clogged orifice inside the lifter body.
If you can confirm your Mitsu does use HLA's, there is also an air purging procedure in the factory shop manual which helps to bleed out air from the lifters to eliminate any excess noise. This can be a helpful troubleshooting aid.
DF
[ March 10, 2003, 09:56 PM: Message edited by: diesel_fan ]