Just got a Romanian 75 round drum and it has quite a bit of oil on it. I wiped down as much as I could but couldn't get the inside very good. What's the best way to clean the excess oil out? It's a rear load drum by the way. Thanks.

Drop it in a bucket (preferably one with a top) add 1/12 gallons of coleman white gas, and let stand for a couple hours, Burp the lid to release expanding fumes well away from fire or flame.

Replace top and swish it around for a minute or two.

Let stand another hour or so and retrieve the drum after pouring your coleman fuel back in the cans  for future use of cosmoline/oil cutting.

I'd use a good degreaser like White Lightning or Simple Green then rinse it out with some hot soapy water. Or just some Dawn dishwashing soap and hot water may get it. So far I've been impressed with my Romy drum and I'm using it more than I thought I would. Zero failures and it's compact enough for using with a bipod on a tabletop.

Whats the benifate to cleaning out the oil? Is it just to prevent dirt from building up? I have one thats primarily a range mag and I just shoot it as is.

I'm assuming the OP just had excessive oil from the factory, mine just had a very light coating of oil inside & out so I just wiped down the outside of mine. Not sure what the accepted maintenance of a drum is, but I should have added in my above post to spray the inside down with a light oil like Remington oil after cleaning. Probably not needed for reliable operation but I'd do it just as a rust inhibitor. With brass casings you don't want to get excessive oil or lube on your ammo as it doesn't allow for normal chamber expansion, not sure if this rule applies to steel case as well.

Yeah it was pretty much dripping when I got it. Wiped most of it out. Thanks for the responses guys.