Low temp R-507 oil return issues
Just installed a R-507 Low temp Beacon II system and am having a oil return issue. The condenser unit is mounted about 3-4 feet above the evaporator on the roof and I have a u-trap on the outlet of the evaporator. After running the system for a day the sight glass on the semi-hermetic was at about 1/8-1/4 oil level running so I added about 5-7 oz of oil and it came back up to half full sight glass.
Now fast forward a week and the sight glass is down in the 1/4 full level again. I'm running about 6 degrees of evap superheat with a total superheat at the compressor of about 35-40 degrees. The sub-cooling hangs around 5-7 degrees and for the most part everything looks good except the oil level. I don't want to keep putting oil in and have the system burp it up later on and cause a serious problem but worry about long term damage to the compressor.
Being its a 2HP semi-hermetic it just uses the splasher system so have no way of checking oil pressure aside from just the sight glass.
The system is a Heatcraft Beacon II (climate control label). The evaporator is a LSF090 and the condenser CSS-021L6 with a EAVA-021E Copleland semi-hermetic.
I've tried running extra defrosts going from the preset four to six and it still hasn't made the oil "burp" back into the compressor. Its been pretty mild weather here with highs in the high 70s and lows in the 50s and 60s. I tried blocking off the condenser to bring up the head pressure for a bit to see if that would get the oil back but I didn't want to do it too long being its a air-cooled semi-hermetic.
So my questions are, first I know Copeland specs 1/2 a sight glass and up to a 3/4 sight glass but is 1/4 going to hurt it? I've gotten answers from yes it will to 1/4 to 3/4 is good.
Being my suction riser is only 3-4 feet at best should I reduce it from the 7/8" to maybe a 5/8"? As a random thought would switching to R-404A help oil return because of its R-134a content?
I've never had a oil return issue with such a small system so kinda at a loss for ideas and appreciate any you guys can offer.