New member here and I apologize for the length of my first thread. My '94 Mercedes E420 had run its entire life on Mobil 1 15W-50, without issue. (I am in TX, so the ambient temperature can be quite high, hence the high viscosity.) Then I discovered that Mobil had quietly switched from Group IV and V base stock to Group III base stock, which I viewed as both hypocritical and underhanded. Hypocritical, because Mobil sued Castrol for calling their Group III base stock oils "synthetic" while Mobil did the exact same thing after the ruling was in Castrol's favor; and underhanded, because they did so quietly, without informing their customer base.
I switched to Royal Purple HPS 20W-50. My second oil change with Royal Purple was at a 7,200 mile interval, which is significantly longer than my standard 6,000-mile oil change interval. ~ 3 weeks prior to this last oil change, I noticed that my oil pressure gauge was acting wonky (rapid fluctuations, sometimes pegged at max - even at idle, or 1 bar when cruising at 80 mph, when it should have been at 3 bars). Today, I took my vehicle into my Mercedes mechanic for him to check out since I thought it might be an oil pressure sending unit issue. I also had him change the oil and filter (Mann canister filter) with Royal Purple HPS 20W-50. The mechanic said that after the oil change the oil pressure gauge was reading/operating properly, which I confirmed, albeit only briefly. Irrespective of the oil pressure gauge/sending unit, my mechanic stated that there is not an actual oil pressure issue.
My concern is two-fold 1) my mechanic said that the oil filter was showing the very first signs of sludge build-up (filminess), which I thought was unusual for a top-tier motor oil such as Royal Purple - even at 7,200 a mile oil change interval, and 2) the oil seems to have interfered with my oil pressure sending unit. Since my vehicle has 240,000+ miles on it, he suggested that I switch to Valvoline 20W50 Extended mileage conventional (Mineral) oil. I found that Valvoline does make a synthetic blend Max Life in 20W50, but they do not seem to have a full synthetic "high mileage" oil in either 15W50 or 20W50. While I understand that synthetic motor oil is not required for my 119 engine, I don't really want to switch back to a conventional motor oil since the vehicle has run synthetic oil for its entire life.
Now I'm confused about what motor oil I should use. While I haven't lost faith in Royal Purple just yet, I am concerned about the possible sludge issue - even if it was just beginning to present. The oil pressure interference is another issue that may or may not be attributable to Royal Purple.
I would like to use a synthetic oil that is, reasonable priced ~$8.00/qt., relatively easy to source and is a genuine synthetic oil (group IV or V base stock). Liqui Moly and Motul are difficult to find from a US distributor, especially for a decent price, but can be ordered online if I pay a hefty shipping fee. Red Line is fairly expensive. I'm not sure what other 15w50 and 20W50 oils I should consider and I'm not sure if a High Mileage formulation is beneficial or necessary. I am not interested in using Amsoil due to a previous bad experience… or perhaps I should stick with RP?
Any recommendations based upon experience would be appreciated. Thank you.
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Napa carries Liqui-Moly at a fair price. And if your particular store doesn't carry a particular L-M product you're looking for, they can order it with no problem 
M1 0w40 from Walmart is most liokely the cheapest, easiest-to-find oil I'd use on that Benz
