For an oil enema you can use mineral oil or a food grade oil like olive oil and there is little, if any, difference.
If you give a reasonable quantity, say 8 Ounces or less, when lying in bed just before going to sleep, it's likely most people will retain it overnight without leakage, but it's always wise to wear a diaper or put down a Chux pad... just in case. Always remember, if you have had a mineral oil enema and feel a need to fart, much more than gas may well come out. Any oil that comes out is likely to be stinky too.
As to feeling "the weight" mineral oil (or olive oil, for that matter) is lighter (less dense) than water. It should not feel heavier than an equal amount of water.
The only reason I can think of for the feelings is the oil makes the inside of your colon very slippery (that's why it's given and why it makes people go) and any poo may thus slide down and you'd feel more pressure to go.
One thing that definitely does feel much heavier is the Barium given by doctors for an upper or lower GI series x-ray exam. Many years ago, I had an upper GI and had to drink a big cup of the stuff. The doctor told me after the exam to have dinner in the evening and then take a large dose of Milk of Magnesia to get the Barium out. I didn't get a strong urge to poo, but was astounded at how heavy the poo in me felt. The laxative had made it quite soft, but it almost fell out of me due to the higher density. I was fairly surprised that the poo was also almost white in color.
If you don't put in an excessive amount of oil, most people should not leak much. A mini pad should be sufficient to protect your underwear.
I'm not a fan of an SSE after an oil enema, unless you're very constipated and don't have a complete poo from the oil alone. If you do have a complete poo after getting the oil and pass both hard and soft, I'd just stop there. The remaining oil will soften and lubricate for several days so rebound constipation is less likely. In fact, when I used to go for colonics with one particular therapist, she'd give an oil enema of about 2 ounces and have me in several positions thereafter for a few minutes to distribute the oil at the end of every session. Usually, nothing would then happen for three or four days, until I'd have a soft and easy poo. It worked very well. The oil 'kept me going'.