Marvel is still good as ever. Question whether or not you need it still is one that will always be asked. Some same that modern motor oils have all the detergent they need to keep modern engines clean, but I will say, I still use it in "used" cars that I purchase. Particularly those that were previously neglected. Due to possible sludge or other varnish build up, I'll do an oil change adding 20% MMO to the crankcase, along with the motor oil. In my most recent 88 Dodge Aries purchase, it took 1 pint of MMO, and 3.5 quarts of oil, for a 4 qt capacity crankcase. Ran that oil for 3000 miles, and the oil came out extremely dark. I had to top off the oil during that 3k twice, using about 1 qt of top off oil total. At the 3k mark, drained that oil and again, added another 1 pt of MMO and 3.5 qts of motor oil, and for that 3k OCI - no oil loss at all. Also noticed the oil was a lot lighter, not as "black" as previous.
Have also been adding about 4-5 oz. every fill up - the bottle states 4 oz. per 10 gallons, or 2 oz. of MMO per 5 gallons...this old gem is runnin like a top :)