I have driven my share of old engines, mostly 240s and 700s series Volvos and one thing that amazes me about blowing oil seals is that some people think that by pushing that seal back in and securing it to the head or valve cover will stop it from happening again, but it may make it worse.
Just over a year ago I took over a heavy miler 850GLT, great little runner with about 340,000 km on the dash, (the odometer gear was found on the floor, so someone had pulled it and it wasn't showing any new kms.) Lately, it started to blow the cam seal. I noted that on pulling it out it had been worked on before with lots of orange RTV (Room Temperature Vulcanizing) sealant. So the issue had been happening for a while then. I picked up a new seal and popped it in. Went in nicely but it popped out in a matter of minutes. So there must have been another problem. So I took the oil cap off - refitted the seal, then started the engine and was getting lots of blow off from the valve cover. i.e. lots of back pressure. So I know that the seal is not the problem it is the crank case ventilation system.
In order to stop my seal blowing out, I have pulled the dipstick up about 1 inch to let out the back pressure. Once the engine is warm, I can see a spout of mixed gases coming out through the dipstick tube- a clear sign that I have a blocked PCV (positive crankcase ventilation system). Once it is warm enough this spring, I will change out the system and change my oil.