When is it necessary to remove and clean oil tank on RV12?

Since owning my RV 12 the last two years I have only used unleaded fuel and therefore changed the oil as recommended every 50 hours. I use Aeroshell Sport oil.

Just completed first x-country of about 20 hours using 100LL . I used Decalin additive with every fuel load in the amount recommended.

I’m planning on changing oil and filter now. Under this scenario do I need to clean oil tank?

Based on my experience I suspect that you will find some very small amount of gray lead slime / sludge in the bottom of your oil tank -- nothing to obsess over however. Cleaning out the oil tank on an RV-12 is not a big deal as there is typically no need to purge the oil system after the tank has been cleaned -- just remove the top oil line connections, remove the top of the oil tank and innards, and inspect/clean the tank in situ. Since this is apparently your first prolonged use of 100LL it might prove a good learning experience to inspect the tank and see for yourself the extent of sludge build-up.

What Dave said, excellent advise.