I bought a S55 AMG semi recently it’s been pretty heavily modified. One of the modifications is a split cooling kit, there is oil in the after market reservoir. I can’t find anything online on what could cause this. Any suggestions are welcome

Sounds like a really cool build and beautiful car. Do you have any pictures you can post? Would love to see the set-up.

I'm personally enticed by the MBH Long Tubes.

There are a few different scenarios that can cause the milky substance. Pictures would be a bit of help. Is it a milky substance or more of a "Strawberry Milkshake" color?

Right off the bat the most obvious answer is head gaskets, but before we jump to conclusions theres a few steps you can take.

First I would pressurize the coolant system and make sure its not leaking down. You can buy a kit online. You may get lucky and find the issue right away with spewing coolant.

I would make sure the split cooling system has been routed properly, however even if it wasn't it wouldn't cause an oil/coolant mixture to occur i don't believe.

The transmission cooler is built into the radiator on the end tank and that failure will cause the oil/coolant to mix, causing a Strawberry Milkshake color.

You can also rebuild the engine oil cooler with fresh gaskets as this is a common failure point. Very easy job to do and a Preventive maintenance one at that. Only special tool you need is a large socket (i forget the size, simple google search) and the gasket kit from FCP is cheap.

I would then drain the oil and coolant and replace with new.

Start there and report back with what you find. If you want to take it a step farther if all checks out you can do a compression test on each cylinder to make sure that checks out as well.

If you are still getting mixing coolant/oil after that you can dig farther into the cooling system, which runs through the supercharger as well as taking off the surge tanks to check for coolant inside those as well.

Good luck

- Joe