Because if it did, they'd advertise it like Mobil does/did
They are careful not to advertise that, and I think
@The Critic has likely hit on why that is.
Cleaning requires different chemistry than keeping things clean. You can have a pretty neutral base oil blend and a healthy detergent package and keep things very clean (which is what Shell is claiming). On the other hand, to actually remove existing deposits you need to use:
- Solvent (obviously not going to work as part of a full formulated engine oil)
- Polar additive/base (this is what HPL uses, AutoRX...etc)
Bases capable of removing existing deposits are AN's and esters, both of which are expensive. AN's have the added challenge of having very low VI's, which means you need to blend them with PAO typically to get a 0W-xx Winter grade. Mobil's original "Tri-Syn" formula was PAO + esters and AN's. When they introduced Group III, they stepped away from that single approach and started mixing it up based on cost and performance targets, so you'd get oils like M1 EP 0W-20 with a majority PAO base and then probably just a splash of ester to meet the solvency, while its sister product in 5W-20 would be totally Group III with no PAO and likely little to no AN or ester content.
Their 0W-40, when it was claimed to clean (it doesn't claim that anymore from what I've seen) likely had either AN's, esters, or both in it.
As
@The Critic noted, introducing cleaning creates potential for serious problems like plugging up filters because Joe Average isn't going to be short-changing the filter in anticipation of rapid loading due to cleaning taking place. Also, the detergent and dispersant package has limited holding capacity and an oil that's cleaning is going to hit that threshold much quicker than one that isn't. Ergo, there is risk associated with making an oil capable of removing existing deposits.