In the air cooled VW world, cleaning tanks out involve cleaners and some kind of abrasive objects. I have used diesel and a length of steel chain to clean tractor tanks. It is flat out work, shaking a bulky tank around. Having teen age boys, I put them to work...

__________________
95 E300D working out the kinks
77 300D, 227k, station car
83 300CD 370k, gone away
89 190E 2.6- 335k, no more
79 VW FI Bus- 145k miles, summer driver
59 VW Beetle ragtop- 175k miles
12 VW Jetta- 160k miles