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It's not hard to fix. There are lots of writeups in this forum on how to do it. If it's coming from one of the gaskets then you can just change that but if it's coming from the tube then you'll need to rebuild it. It's easy.
I like 5w40 rotella synthetic in the winter myself.
Its not to hard to reseal oil cooler. Just drain coolant, remove cooler, catch the oil that drains out, and replace a few seals and reinstall.
I run rotella 15w40 year round with no issues but I also plug my engine in all the time and after i start it I let it warm up for 10-15 mins before rolling down the road